drm/i915: Clean up rotation DSPCNTR/DVSCNTR/etc. setup

Move the plane control register rotation setup away from the
coordinate munging code. This will result in neater looking
code once we add reflection support for CHV.

v2: Drop the BIT(), drop some usless parens,

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/1479142440-25283-3-git-send-email-ville.syrjala@linux.intel.com