drm/i915: Fix assert in i915_ggtt_pin

Use lockdep_assert_not_held to simplify and correct the code. Otherwise
false positive are hit if lock state is uknown like after a previous
taint.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429140757.651406-1-tvrtko.ursulin@linux.intel.com