accel/ivpu: Fix for missing lock around drm_gem_shmem_vmap()

drm_gem_shmem_vmap/vunmap requires dma resv lock to be held.
This was missed during conversion to shmem helper.

Fixes: 8d88e4cd ("accel/ivpu: Use GEM shmem helper for all buffers")
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240115134434.493839-6-jacek.lawrynowicz@linux.intel.com