The ATI Rage 128 driver has been the only user of ATI PCI GART code
since Radeon dropped UMS support in commit 8333f607 ("drm/radeon:
remove UMS support"). Clean up the drm top level directory, Kconfig and
Makefile by making ati_pcigart.[ch] part of r128. Drop the
CONFIG_DRM_ATI_PCIGART config option made redundant by the change.

This reduces drm.ko module size slightly when legacy drivers are
enabled, and moves the baggage to r128.ko instead.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119100536.12024-1-jani.nikula@intel.com