drm/msm/gem: Fix double resv lock aquire

Since commit 79e2cf2e ("drm/gem: Take reservation lock for vmap/vunmap
operations"), the resv lock is already held in the prime vmap path, so
don't try to grab it again.

v2: This applies to vunmap path as well
v3: Fix fixes commit

Fixes: 79e2cf2e

 ("drm/gem: Take reservation lock for vmap/vunmap operations")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Christian König <christian.koenig@amd.com>
Patchwork: https://patchwork.freedesktop.org/patch/576642/