drm: Allow const struct drm_driver

It's nice if a big function/ioctl table like this is const. Only
downside here is that we need a few more #ifdef to paper over the
differences when CONFIG_DRM_LEGACY is enabled. Maybe provides more
motivation to sunset that horror show :-)

v2:
- Fix super important checkpatch warning (Sam)
- Update the kerneldoc example too (Sam)

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-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/20201104100425.1922351-4-daniel.vetter@ffwll.ch