drm/i915: Remove unused underrun irq/reporting bits

drm/i915: Remove unused underrun irq/reporting bits

Underrun recovery was defeatured and was never brought into usage.
Thus we can remove the underrun recovery interrupt/reporting
register bits and related logic introduced to detect/report soft,
hard, port underruns.

Essentially this is a revert of the commit 8bcc0840cf7c
("drm/i915/xelpd: Enhanced pipe underrun reporting") which originally
added this functionality. Also note that PIPE_STATUS_UNDERRUN bit in
PIPESTATUS still stays relevant but we would move back to not
clearing this sticky bit as we are not using any information from
this register.

v2: Extend commit message to add more details (Matt Roper)
v3: Fix the old commit mention in commit message

Signed-off-by: Sai Teja Pottumuttu <sai.teja.pottumuttu@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241015080503.3521063-1-sai.teja.pottumuttu@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>