drm/i915/pmu: Fix enable count array size and bounds checking

drm/i915/pmu: Fix enable count array size and bounds checking

Enable count array is supposed to have one counter for each possible
engine sampler. As such, array sizing and bounds checking is not correct
and would blow up the asserts if more samplers were added.

No ill-effect in the current code base but lets fix it for correctness.

At the same time tidy the assert for readability and robustness.

v2:
 * One check per assert. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: b46a33e271ed ("drm/i915/pmu: Expose a PMU interface for perf queries")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190205130353.21105-1-tvrtko.ursulin@linux.intel.com
(cherry picked from commit 26a11deea685b41a43edb513194718aa1f461c9a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>