drm: Check the fb size against the adjusted v/hdisplay for stereo modes

Some stereo modes, like frame packing, need a larger CRTC viewport than
the "natural" underlying 2D mode and thus drm_crtc_check_viewport()
needs to query the adjusted mode to use the correct h/vdisplay.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>