drm: Reject mode set with current fb if no current fb is bound

When doing a mode set with the special fb id -1, reject the mode set if
no fb is currently bound to the crtc.

Also remove the pointless list traversal to find the current crtc based
on the current crtc :)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>