drm/amdgpu: drop WARN_ON in amdgpu_gart_bind/unbind

NULL pointer check has guarded it already.

calltrace:
amdgpu_ttm_gart_bind+0x49/0xa0 [amdgpu]
amdgpu_ttm_alloc_gart+0x13f/0x180 [amdgpu]
amdgpu_bo_create_reserved+0x139/0x2c0 [amdgpu]
? amdgpu_ttm_debugfs_init+0x120/0x120 [amdgpu]
amdgpu_bo_create_kernel+0x17/0x80 [amdgpu]
amdgpu_ttm_init+0x542/0x5e0 [amdgpu]

Fixes: 1b08dfb8 ("drm/amdgpu: remove gart.ready flag")
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c

@@ -161,7 +161,7 @@ void amdgpu_gart_unbind(struct amdgpu_device *adev, uint64_t offset,
 	uint64_t flags = 0;
 	int idx;
 
-	if (WARN_ON(!adev->gart.ptr))
+	if (!adev->gart.ptr)
 		return;
 
 	if (!drm_dev_enter(adev_to_drm(adev), &idx))
@@ -241,7 +241,7 @@ void amdgpu_gart_bind(struct amdgpu_device *adev, uint64_t offset,
 		     int pages, dma_addr_t *dma_addr,
 		     uint64_t flags)
 {
-	if (WARN_ON(!adev->gart.ptr))
+	if (!adev->gart.ptr)
 		return;
 
 	amdgpu_gart_map(adev, offset, pages, dma_addr, flags, adev->gart.ptr);