drm/i915/watermark: Check bounds for scaler_users for dsc prefill latency

Currently, during the computation of global watermarks, the latency for
each scaler user is calculated to compute the DSC prefill latency.
At this point, the number of scaler users can exceed the number of
supported scalers, which is checked later in intel_atomic_setup_scalers().

This can cause issues when the number of scaler users exceeds the number
of supported scalers.

While checking for DSC prefill, ensure that the number of scaler users does
not exceed the number of supported scalers.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/4341


Fixes: a9b14af9 ("drm/i915/dsc: Check if vblank is sufficient for dsc prefill")
Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250227034106.1638203-1-ankit.k.nautiyal@intel.com