drm/i915/perf: reuse intel_lrc ctx regs macro

Abstract the context image access a bit.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180813080218.28994-3-tvrtko.ursulin@linux.intel.com