drm/i915: Fix display pixel format handling

Fix support for all RGB/BGR pixel formats (except the 16:16:16:16 float
format).

Fix intel_init_framebuffer() to match hardware and driver limitations:
* RGB332 is not supported at all
* CI8 is supported
* XRGB1555 & co. are supported on Gen3 and earlier
* XRGB210101010 & co. are supported from Gen4 onwards
* BGR formats are supported from Gen4 onwards
* YUV formats are supported from Gen5 onwards (driver limitation)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>