drm/i915: print out which pixel format we do not support

Otherwise, we are left with pretty bogus message saying that the pixel
format is not supported while leaving the details to the telepatic powers.

v2: use DRM_DEBUG_KMS instead of DRM_ERROR

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>