Screen/Linux: Bugfix: Enable 10 bpc framebuffers on AMD DCE4+ gpu's.

Some stray check for DCE version <= 1.0 was left, preventing this
to work on DCE-4 and later.
Screen on Linux/OSX: Fix AMD DCE-4 detection and mmio handling.

Two hilarous bugs fixed:

a) If DCE4+ was detected, the minimal allowable register access offset was
   set to a well meaning and wrong value. This caused register read/write
   access to regs below that wrong limit to silently fail, because the
   error reporting was non-existent! Hilarious debugging fun ensures!

   -> Set limit to zero to disable it. Too much potential for harm with
      too less benefit.

   -> Improve error reporting to reduce future fun.

b) AMD DCE engine generations are reported in decimal notation, but
   some of the DCE generation specific code matched against hexadecimal
   limits, e.g., 0x40 instead of 40 for detecting DCE-4. This caused
   code to no-op, or wrong code pathes to be taken for a given gpu.

   Affected functionality, which totally malfunctioned in DCE4+:

   Identity lut and color offset loading and checking for pixel
   identity passthrough setup on AMD, aka LoadIdentityClut().

   Dithering control on DCE4+.

   10 bit ARGB2101010 framebuffer setup on DCE4+

a) and b) combined explain following bugs, which are now fixed:

- Inability to configure low level pixelpassthrough on AMD DCE4+ on
  both Linux and OSX with AMD cards.

- Inability to enable 10 bpc framebuffers on DCE4+ on Linux and OSX.

- Inability to auto-detect screen->head mappings via auto-probing:
  This was considered unworkable and is still disabled and untested
  in this commit. However the likely reason it never worked is simply
  these bugs. Need to reenable and retest in future commits.

-> Source code updated, but no mex files included, so we can
   cherry-pick this easily into the 3.0.11 branch.

-> Will also need rebuild for PsychtoolboxKernelDriver on OSX.
Screen for 64-Bit Octave and Matlab on Linux test-rebuilt.

-> Confirms that identity passthrough and 10 bpc framebuffers now
   work on DCE-4.
PsychRTBox.m: Fix for firmware version 5.0

v5.0 firmware doesn't seem to send back acknowledge char 'e' anymore
for event 'e'nable. So don't expect an ack for 'e' on version 5.

Is this a complete fix? I don't know, because that f$#%# firmware
and underlying protocol gets changed over and over to make it a
bit worse with each iteration.