drm/radeon: Don't flush the GART TLB if rdev->gart.ptr == NULL

commit 233709d2 upstream.

This can be the case when the GPU is powered off, e.g. via vgaswitcheroo
or runpm. When the GPU is powered up again, radeon_gart_table_vram_pin
flushes the TLB after setting rdev->gart.ptr to non-NULL.

Fixes panic on powering off R7xx GPUs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61529Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/gpu/drm/radeon/radeon_gart.c

@@ -249,8 +249,10 @@ void radeon_gart_unbind(struct radeon_device *rdev, unsigned offset,
 			}
 		}
 	}
+	if (rdev->gart.ptr) {
 		mb();
 		radeon_gart_tlb_flush(rdev);
+	}
 }
 
 /**
@@ -292,8 +294,10 @@ int radeon_gart_bind(struct radeon_device *rdev, unsigned offset,
 			}
 		}
 	}
+	if (rdev->gart.ptr) {
 		mb();
 		radeon_gart_tlb_flush(rdev);
+	}
 	return 0;
 }