drm/msm/gpu: Don't map command buffers with nr_relocs equal to 0

If a command buffer doesn't have any relocs assigned to it there then
is no need to map it in the kernel address space.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>