drm/i915/pmu: Change bitmask of enabled events to u32

Having it as u64 was a confusing (but harmless) mistake.

Also add some asserts to make sure the internal field does not overflow
in the future.

v2: Fix WARN_ON firing for INTERRUPT event (Umesh)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230519154946.3751971-2-umesh.nerlige.ramappa@intel.com