drm: Compile out legacy chunks from struct drm_device

This means some very few #ifdef in code, but it allows us to
enlist the compiler to make sure this stuff isn't used anymore.

More important, only legacy drivers change drm_device (for the
legacy_dev_list shadow attach management), therefore this is
prep to allow modern drivers to have a const driver struct. Which
is nice, because there's a ton of function pointers in there.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Review-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201104100425.1922351-2-daniel.vetter@ffwll.ch