drm/i915: fixup fb bpp computation in pipe_config_set_bpp

Ville pointed out that my assumption that no unsupported pixel format
can get past the pipe config computation stage to the platform
update_plane callbacks is wrong. The reason is that this function
still checks the old fb->depth value instead of the new pixel_format.

While checking with all the other places that use this I've noticed
that intel_framebuffer_init already has all the platform checks we
need, so replace those checks with a WARN_ON.

Since fb->depth isn't set for YUV pixel formats and since we already
can't create an fb with an rgb layout not support on the running
platform I /think/ this patch doesn't fix any bug.

But it surely looks better!

v2: BGR formats are also only gen4+, so add the corresponding WARN_ON,
too (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>