drm/mm/selftests: fix wrong return type casting

Function prepare_igt_frag() and get_insert_time() were casting
signed value to unsigned value before returning error.
So error check in igt_frag() would not work with unsigned
return value from get_insert_time() compared against negative
value.

Addresses-Coverity: ("Unsigned compared against 0, no effect")
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/370636/