drm/radeon: fix unsigned comparison with 0

Fixes warning because pipe is unsigned long and can never be negtative

vers/gpu/drm/radeon/radeon_kms.c:831:11: warning:
comparison of unsigned expression < 0 is always false [-Wtype-limits]
  if (pipe < 0 || pipe >= rdev->num_crtc) {
drivers/gpu/drm/radeon/radeon_kms.c:857:11: warning:
comparison of unsigned expression < 0 is always false [-Wtype-limits]
  if (pipe < 0 || pipe >= rdev->num_crtc) {

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: ChenTao <chentao107@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>