drm/i915/pmu: Use existing uncore helper to read gpm_timestamp

Use intel_uncore_read64_2x32 to read upper and lower fields of the GPM
timestamp.

v2: Fix compile error

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220427003515.3944267-1-umesh.nerlige.ramappa@intel.com