pvr: Fix signed comparison

pvr: Fix signed comparison

Fix defect reported by Coverity Scan.

Unsigned compared against 0 (NO_EFFECT)
unsigned_compare: This less-than-zero comparison of an unsigned value is never true. val < 0U.
unsigned_conversion: val is converted to an unsigned type because it's compared to an unsigned constant.

Fixes: 480bdff4b52 ("pvr: Add support to process transfer and blit cmds")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23416>