drm/msm: Map command buffers to kernel only if required

Map command buffers to the kernel address space only if
relocs are specified for the submission. This reduces some
overhead on the submission path.

Change-Id: I32ca3c7fe2147c835a328e0c8937b45f2f3d59b9
Signed-off-by: Sushmita Susheelendra <ssusheel@codeaurora.org>