drm: Use a const drm_driver for legacy PCI devices

Now that the legacy PCI support code doesn't need to write to the
drm_driver structure, it can be treated as const through the whole DRM
core, unconditionally. This allows declaring the structure as const in
all drivers, removing one possible attack vector.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>