drm/i915: Remove erroneous i915_is_ggtt check for I915_GEM_OBJECT_UNBIND_VM_TRYLOCK

We changed the locking hierarchy for both ppgtt and ggtt, so both locks
should be trylocked inside i915_gem_object_unbind().

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: bc6f80cc ("drm/i915: Use trylock in shrinker for ggtt on bsw vt-d and bxt, v2.")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429120158.1105318-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com> #irc