drm/i915: Convert i915_reg_read_ioctl to use explicit mmio accessors

No excuse for code located in intel_uncore.c to not use intel_uncore_
helpers.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610120608.15477-2-tvrtko.ursulin@linux.intel.com