drm/amd/display: Fix unsigned comparison with less than zero

drm/amd/display: Fix unsigned comparison with less than zero

The return value from the call to dml21_find_dc_pipes_for_plane() is int.
However, the return value is being assigned to an unsigned int variable
'num_pipes', the condition if(num_pipes <= 0) is not rigorous enough,
so making 'num_pipes' an int.

./drivers/gpu/drm/amd/display/dc/dml2/dml21/dml21_wrapper.c:318:6-15: WARNING: Unsigned expression compared with zero: num_pipes <= 0.
./drivers/gpu/drm/amd/display/dc/dml2/dml21/dml21_wrapper.c:360:6-15: WARNING: Unsigned expression compared with zero: num_pipes <= 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9454
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>