drm/i915: Stop using I915_READ/WRITE in intel_wopcm_init_hw

More legacy mmio accessor removal. We pass in intel_gt explicitly allowing
code to use new intel_uncore_read/write helpers.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-17-tvrtko.ursulin@linux.intel.com