MLK-18848-3: drm: imx: dcss: split vblank irq enable routine

MLK-18848-3: drm: imx: dcss: split vblank irq enable routine

Currently, when enabling/disabling vblank interrupt, we also
enable/disable the CTXLD kick interrupt. Most of the time this is fine,
because when vblank gets disabled user-space does not submit any buffers
and CTXLD kick interrupt is not needed.

There is one case when we actually need to be able to have the CTXLD
kick interrupt enabled: when disabling CRTC. Vblank interrupt, in this
case, is disabled before the crtc_atomic_disable routine is called.
However, we still need CTXLD to push the changes to SUBSAM and DTG.

This patch will create a routine just for enabling/disabling CTXLD kick
interrupt and move the code from vblank routine to the new one.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>