The primary and sprite planes on CHV pipe B support horizontal
mirroring. Expose it to the world.

Sadly the hardware ignores the mirror bit when the rotate bit is
set, so we'll have to reject the 180+X case.

v2: Drop the BIT()
v3: Pass dev_priv instead of dev to IS_CHERRYVIEW()
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479142440-25283-4-git-send-email-ville.syrjala@linux.intel.com