drm/i915: pass dev_priv explicitly to DSPGAMC

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

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/542455204f62182a46fa2cb16ad6b0648c72f612.1716469091.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>