drm/i915: pass dev_priv explicitly to PIPEGCMAX

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

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5fdc8562fe7b8d26e1ec1bb8f9a221348246bbe1.1716469091.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>