drm/i915/pmu: Make PMU sample array two-dimensional

No functional changes but we can remove some unsightly index computation
and read/write functions if we convert the PMU sample array from a
one-dimensional to a two-dimensional array.

v2: Retain read/store helpers (Tvrtko)

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230524215629.97920-3-ashutosh.dixit@intel.com