[Intel-gfx] Use PIPE_CONTROL where possible

[Jesse Barnes]

Recent hardware supports the PIPE_CONTROL command.  It seems to be
necessary for stability on Ironlake at least, and should be a
performance win on other platforms.  This patchset (tested on Ironlake
only) adds support for the PIPE_CONTROL command and changes GEM to use
it by default.

There's a stray FBC cleanup patch in here as well, since I was working
on debugging some leftover DMA mappings at unload.  Of course, since I
was testing on Ironlake, the FBC cleanup has no effect, but it's a good
patch anyway.

Thanks,
Jesse