drm/crtc-helper: no need to check for fb->depth/bpp

... since we already check for fb->pixel_format, which encodes all
this. The other two fields are only for backwards compat of older
drivers (and we might want to look into eventually just killing them).

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>