drm/i915: Store a pointer to the pixel format info for fbc

drm/i915: Store a pointer to the pixel format info for fbc

Rather than store the pixel format and look up the format info as
needed,  let's just store a pointer to the format info directly
and speed up our lookups.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-29-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>