pvr: fix invalid read reported by valgrind

pvr: fix invalid read reported by valgrind

pvr_gpu_upload() can't be used in the case of pvr_gpu_upload_usc() as it expects
the source and destination buffers to be the same size. This isn't the case
because pvr_gpu_upload_usc() adds some padding bytes to the size passed in by
the caller.

Fixes: 547a10f8702 ("pvr: switch pvr_cmd_buffer_alloc_mem to use pvr_bo_suballoc")
Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23185>