drm: adp: Enable vblank interrupts in crtc's .atomic_enable

drm: adp: Enable vblank interrupts in crtc's .atomic_enable

Calling drm_crtc_vblank_on() drm_crtc_helper_funcs' atomic_enable is
expected to enable vblank interrupts. It may have been avoided here to
due to drm_crtc_vblank_get()'s error behavior after
drm_crtc_vblank_reset(). With that fixed in the preceding change the
driver can call drm_crtc_vblank_on() from adp_crtc_atomic_enable().

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Janne Grunau <j@jannau.net>
Link: https://lore.kernel.org/r/20250428-drm_adp_fixes-v2-3-912e081e55d8@jannau.net
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>