drm/vc4: Correct handling of rotation parameter in fkms

One bit within DRM_MODE_ROTATE_MASK will always be set to
determine the base rotation 0/90/180/270, and then REFLECT_X
and REFLECT_Y are on top.

Correct the handling which was assuming that REFLECT_[X|Y]
was instead of ROTATE_x.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>