drm/i915/perf: fix ICL perf register offsets

We got the wrong offsets (could they have changed?). New values were
computed off an error state by looking up the register offset in the
context image as written by the HW.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 1de401c0

 ("drm/i915/perf: enable perf support on ICL")
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610081914.25428-1-lionel.g.landwerlin@intel.com