drm/i915/pch: replace BUG_ON() with drm_WARN_ON()

Avoid BUG_ON(). Replace with drm_WARN_ON().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220830093411.1511040-3-jani.nikula@intel.com