i915/perf: Drop the aging_tail logic in perf OA

On DG2, capturing OA reports while running heavy render workloads
sometimes results in invalid OA reports where 64-byte chunks inside
reports have stale values. Under memory pressure, high OA sampling rates
(13.3 us) and heavy render workload, occasionally, the OA HW TAIL
pointer does not progress as fast as the sampling rate. When these
glitches occur, the TAIL pointer takes approx. 200us to progress.  While
this is expected behavior from the HW perspective, invalid reports are
not expected.

In oa_buffer_check_unlocked(), when we execute the if condition, we are
updating the oa_buffer.tail to the aging tail and then setting pollin
based on this tail value, however, we do not have a chance to rewind and
validate the reports prior to setting pollin. The validation happens
in a subsequent call to oa_buffer_check_unlocked(). If a read occurs
before this validation, then we end up reading reports up until this
oa_buffer.tail value which includes invalid reports. Though found on
DG2, this affects all platforms.

The aging tail logic is no longer necessary since we are explicitly
checking for landed reports.

Start by dropping the aging tail logic.

v2:
- Drop extra blank line
- Add reason to drop aging logic (Ashutosh)
- Add bug links (Ashutosh)
- rename aged_tail to read_tail
- Squash patches 3 and 1

v3: (Ashutosh)
- Remove extra spaces
- Remove gtt_offset from the pollin calculation
- s/Bug:/Link/ in commit message (checkpatch)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7484
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7757
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/20230605193923.1836048-2-umesh.nerlige.ramappa@intel.com