drm: zynq_drm: drv: Allow disabling vblank

Fix to disable vblank properly. In this way, vblank is disabled
for last drm_vblank_put(), and re-enabled for the first drm_vblank_get().
Otherwise, resetting vtc from pipeline re-configuration disables vtc
interrupt.

Signed-off-by: hyun woo kwon <hyunk@xilinx.com>
drm: zynq_drm: crtc: Add a common function set_base()

Add a common function _zynq_drm_crtc_mode_set_base() to be shared by
zynq_drm_crtc_mode_set_base() and zynq_drm_crtc_page_flip().

Signed-off-by: hyun woo kwon <hyunk@xilinx.com>