drm: renesas: shmobile: Turn vblank on/off when enabling/disabling CRTC

The DRM core vblank handling mechanism requires drivers to forcefully
turn vblank reporting off when disabling the CRTC, and to restore the
vblank reporting status when enabling the CRTC.
Implement this using the drm_crtc_vblank_{on,off}() helpers.

Note that drm_crtc_vblank_off() must be called at startup to synchronize
the state of the vblank core code with the hardware, which is initially
disabled.  This is performed at CRTC creation time, requiring vertical
blank initialization to be moved before creating CRTCs.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e5833e5706b7792bfca8e6e56fc154a7c3e0574f.1694767209.git.geert+renesas@glider.be