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

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=61529
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Michel Dänzer <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/radeon/radeon_gart.c

···
 			}
 		}
 	}
-	mb();
-	radeon_gart_tlb_flush(rdev);
+	if (rdev->gart.ptr) {
+		mb();
+		radeon_gart_tlb_flush(rdev);
+	}
 }
 
 /**
···
 			page_base += RADEON_GPU_PAGE_SIZE;
 		}
 	}
-	mb();
-	radeon_gart_tlb_flush(rdev);
+	if (rdev->gart.ptr) {
+		mb();
+		radeon_gart_tlb_flush(rdev);
+	}
 	return 0;
 }