Enable VGA output when settings bochs-specific mode

When used from OVMF+CSM, we got no video output. It appears that we were
never enabling the display output except when configuring a text mode.
Which never happens, in the OVMF+CSM case.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Enable VGA output when setting Cirrus-specific mode

This patch does the same for Cirrus as David's following patch for bochs,
originally posted under
<http://www.seabios.org/pipermail/seabios/2013-February/005434.html>:

  Enable VGA output when settings bochs-specific mode

  When used from OVMF+CSM, we got no video output. It appears that we were
  never enabling the display output except when configuring a text mode.
  Which never happens, in the OVMF+CSM case.

In my testing on RHEL-6.3 with OVMF -D CSM_ENABLE / CONFIG_CSM bios.bin /
CONFIG_QEMU vgabios.bin, using Cirrus, VESA mode 0x115 is selected (Direct
Color, 800x600x24).

According to <http://www.osdever.net/FreeVGA/vga/attrreg.htm>,

  cirrus_switch_mode()
    stdvga_attr_mask()

currently keeps/sets the "Attribute Controller Graphics Enable" bit set in
the "Attribute Mode Control Register". When invoked from OVMF+CSM, that is
not enough however, so let's do the same as for Bochs:

  stdvga_attrindex_write(0x20);

which corresponds to setting the "Palette Address Source" bit in the
"Attribute Address Register":

  "This bit is set to 0 to load color values to the registers in the
  internal palette. It is set to 1 for normal operation of the attribute
  controller. [...]"

clext_set_mode()
  stdvga_set_mode() -- for regular modes
    stdvga_attrindex_write() -- existing call
  cirrus_switch_mode() -- for Cirrus modes
    stdvga_attrindex_write() -- call added by this patch

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>