drm/i915/perf: replace .reg accesses with i915_mmio_reg_offset

This replaces accesses to the reg field of the i915_reg_t structure
with the i915_mmio_reg_offset() inline function.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Ewelina Musial <ewelina.musial@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113233455.12085-2-lionel.g.landwerlin@intel.com