drm: atmel_hlcdc: Enable the crtc vblank prior to crtc usage.

'commit eec44d44 ("drm/atmel: Use drm_atomic_helper_commit")'
removed the home-grown handling of atomic commits and exposed an issue
in the crtc atomic commit handling where vblank is expected to be
enabled but hasn't yet, causing kernel warnings during boot.  This patch
cleans up the crtc vblank handling thus removing the warning on boot.

Fixes: eec44d44 ("drm/atmel: Use drm_atomic_helper_commit")

Signed-off-by: Dan Sneddon <dan.sneddon@microchip.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602160846.5013-1-dan.sneddon@microchip.com