drm/i915: Fix gen3 physical cursor alignment requirements

Bspec tells us that gen3 platforms need 4KiB alignment for CURBASE
rather than the 256 byte alignment required by i85x. Let's fix that
and pull the code to determine the correct alignment to a helper
function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-13-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>