drm/v3d: Skip debugfs dumping GCA on platforms without GCA.

Fixes an oops reading this debugfs entry on BCM7278.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928232126.4332-4-eric@anholt.net


Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Cc: <stable@vger.kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>