drm/i915: Use & instead if == to check for rotations

Using == to check for 180 degree rotation only works as long as the
reflection bits aren't set. That will change soon enough for CHV, so
let's stop doing things the wrong way.

v2: Drop the BIT()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479142440-25283-2-git-send-email-ville.syrjala@linux.intel.com