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>

drivers/gpu/drm/radeon/radeon_kms.c

@@ -828,7 +828,7 @@ int radeon_enable_vblank_kms(struct drm_crtc *crtc)
 	unsigned long irqflags;
 	int r;
 
-	if (pipe < 0 || pipe >= rdev->num_crtc) {
+	if (pipe >= rdev->num_crtc) {
 		DRM_ERROR("Invalid crtc %d\n", pipe);
 		return -EINVAL;
 	}
@@ -854,7 +854,7 @@ void radeon_disable_vblank_kms(struct drm_crtc *crtc)
 	struct radeon_device *rdev = dev->dev_private;
 	unsigned long irqflags;
 
-	if (pipe < 0 || pipe >= rdev->num_crtc) {
+	if (pipe >= rdev->num_crtc) {
 		DRM_ERROR("Invalid crtc %d\n", pipe);
 		return;
 	}