drm/i915: Refactor CURBASE calculation

The remaining cursor base address calculations are spread
around into several different locations. Just pull it all
into one place.

v2: Don't pass intel_plane as we don't really need it

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/20170327185546.2977-4-ville.syrjala@linux.intel.com