drm/msm: fix unsigned comparison with less than zero

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: 25fdd593 ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>