drm/i915: pass dev_priv explicitly to PRIMCNSTALPHA

Avoid the implicit dev_priv local variable use, and pass dev_priv
explicitly to the PRIMCNSTALPHA register macro.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/783477b86f4d53849775cbf690bb8b9042792a66.1716469091.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>