AlphaMask: add missing error check.

It's possible to allocate the bitmap object, then fail allocating the
bits. The WebKit testsuite is a way to trigger this, and it crashed
app_server.
Use GCC builtins for byte-swapping. Fixes #10800.

* Introduced in gcc-4.3 for at least Intel platforms
* On ARM, full support added in gcc-4.8
* Other platforms untested, left as-is
* This introduces a breaking change to the ABI for gcc4
radeon_hd: Rework PLL picking code

* Rewrote to be more efficient and more accurate.
* Fix a DP bug where we didn't choose the correct
  external PLL.
radeon_hd: Fix comment typo, no functional change