drm/vc4: Use drm_malloc_ab to fix large rendering jobs.

If you exceeded the size that kmalloc would return, you'd get a dmesg
warning and a return from the job submit.  We can handle much
allocations with vmalloc, and drm_malloc_ab makes that decision.

Fixes failure in piglit's scissor-many.

Signed-off-by: Eric Anholt <eric@anholt.net>