drm/xe/vf: Fix register value lookup

We should use the number of actual entries stored in the runtime
register buffer, not the maximum number of entries that this buffer
can hold, otherwise bsearch() may fail and we may miss the data and
wrongly report unexpected access to some registers.

Fixes: 4edadc41 ("drm/xe/vf: Use register values obtained from the PF")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718203155.486-1-michal.wajdeczko@intel.com