drm/i915: Replace GFX_MODE_GEN7 with RING_MODE_GEN7

It's preferable to use parameterized register macros where possible.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111051600.3429104-6-matthew.d.roper@intel.com