drm/amdgpu: drop WARN_ON in amdgpu_gart_bind/unbind

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: 0b4aa0d820e342 ("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>