drm: Factor out common CRTC viewport checking code

Both setcrtc and page_flip are checking that the framebuffer is big
enough for the defined crtc viewport (x, y, hdisplay, vdisplay). Factor
that code out in a single function.

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>