msm: kgsl: Avoid dynamically allocating small command buffers

Most command buffers here are rather small (fewer than 256 words); it's
a waste of time to dynamically allocate memory for such a small buffer
when it could easily fit on the stack.

Conditionally using an on-stack command buffer when the size is small
enough eliminates the need for using a dynamically-allocated buffer most
of the time, reducing GPU command submission latency.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>