drm/i915: Introduce i9xx_check_plane_surface()

Extract the primary plane surfae offset/x/y calculations for
pre-SKL platforms into a common function, and call it during the
atomic check phase to reduce the amount of stuff we have to do
during the commit phase. SKL is already doing this.

v2: Update the comment about the rotation adjustments to
    match the code better (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323192712.30682-5-ville.syrjala@linux.intel.com