drm/msm: fix unsigned comparison with less than zero

drm/msm: fix unsigned comparison with less than zero

[ Upstream commit dfdb3be43ef1195c491e6c3760b922acb52e3575 ]

The return from the call to _mixer_stages can be a negative error
code however this is being assigned to an unsigned variable 'stages'
hence the check is always false. Fix this by making 'stages' an
int.

Detected by Coccinelle ("Unsigned expression compared with zero:
stages < 0")

Fixes: 25fdd5933e4c ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>