drm: Be more paranoid with integer overflows

Make sure 'width * cpp' and 'height * pitch + offset' don't exceed
UINT_MAX.

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>