drm: drop README.drm, ancient scrolls

This stuff is ancient, we have docs now in the kernel,
lets just drop it.

Pointed out by Glenn

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>