drm/qxl: add mode/framebuffer check functions

Add a helper functions to check video modes.  Also add a helper to check
framebuffer buffer objects, using the former for consistency.  That way
we should not fail in qxl_primary_atomic_check() because video modes
which are too big will not be added to the mode list in the first place.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-21-kraxel@redhat.com