radesonsi: Remove unsigned comparison to zero.

radesonsi: Remove unsigned comparison to zero.

cbuf was changed to unsigned in commit 3fec2f67c3d9 ("radeonsi:
compact MRTs to save PS export memory space").

Fix defect reported by Coverity Scan.

Macro compares unsigned to 0 (NO_EFFECT)
unsigned_compare: This greater-than-or-equal-to-zero comparison of
an unsigned value is always true. cbuf >= 0U.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6739>