egl/sl: use kms_swrast with vgem instead of a random GPU

egl/sl: use kms_swrast with vgem instead of a random GPU

VGEM and kms_swrast were introduced to work with one another.

All we do is CPU rendering to dumb buffers. There is no reason to carve
out GPU memory, increasing the memory pressure on a device that could
make a better use of it.

Note:
 - The original code did not work out of the box, since the dumb buffer
ioctls are not exposed to render nodes.
 - This requires libdrm commit 3df8a7f0 ("xf86drm: fallback to MODALIAS
for OF less platform devices")
 - The non-kms, swrast is unaffected by this change.

v2:
 - elaborate what and how is/isn't working (Eric)
 - simplify driver_name handling (Eric)

v3:
 - move node_type outside of the loop (Eric)
 - kill no longer needed DRM_RENDER_DEV_NAME define

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>