drm/amdkfd: fix uint32 variable compared to less than zero

Currently the call to kfd_process_gpuidx_from_gpuid is returning an
int value and this is being assigned to a uint32_t variable gpuidx
and this is being checked for a negative error return which is always
going to be false. Fix this by making gpuidx an int32_t. This makes
gpuidx also type consistent with the use of gpuidx from the callers.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: cda0f85b ("drm/amdkfd: refine migration policy with xnack on")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>