drm/tilcdc: make frame_done interrupt active at all times

The frame_done interrupt was only being enabled when the vsync
interrupts were being enabled by DRM.  However the frame_done is
used to determine if the LCD controller has successfully completed
the raster_enable, raster_disable commands and the vsync interrupts
are not always enabled during these operations.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>