drm/r128: make ATI PCI GART part of its only user, r128

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