drm/i915: Improve debug print in vm_fault_ttm

Print the error code returned by __i915_ttm_migrate()
for better debuggability.

v2: Fix kernel test robot warning.
v3: Fix dim checkpatch warning.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/6889


Acked-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220923143730.13498-1-nirmoy.das@intel.com