drm/xe: Eliminate usage of TIMESTAMP_OVERRIDE

drm/xe: Eliminate usage of TIMESTAMP_OVERRIDE

Recent discussions with the hardware architects have revealed that
the TIMESTAMP_OVERRIDE register is never expected to hold a valid/useful
value on production hardware.  That register would only get used by
hardware workarounds (although there are none that use it today) or
during early internal hardware testing.

Due to lack of documentation it's not clear exactly what the driver
should be doing if CTC_MODE[0] is set (or even whether that's a setting
that would ever be encountered on real hardware), but it's definitely
not what Xe and i915 have been doing.  So drop the incorrect code trying
to use TIMESTAMP_REGISTER.  If the driver does encounter CTC_MODE[0] in
the wild, we'll print a warning and just continue trying to use the
crystal clock frequency since that's probably less incorrect than what
we're doing today.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250225224908.1671554-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>