drm/i915: framebuffer compression for GM45+

Add support for framebuffer compression on GM45 and above.  Removes
some unnecessary I915_HAS_FBC checks as well (this is now part of the
FBC display function).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>