drm/tilcdc: Turn raster off in crtc reset, if it was on in the HW

This forces the HW to be in sync with the empty state. This should
help with the problem when LCDC is already enabled (e.g. by the
bootloader) at the initialization phase and the enable fails when a
new mode is applied.

Signed-off-by: Jyri Sarha <jsarha@ti.com>