drm/tilcdc: Fix interrupt enable/disable code for version 2 tilcdc

Fix interrupt enable/disable code for version 2 tilcdc. In version 2
tilcdc there is a separate register for disabling interrupts. Writing
0 to enable registers bits does not have any effect. The interrupt
clear register works the same way, writing 1 to specific bit disables
the interrupt and writing 0 does not have any effect.

The "bug" that is fixed here does not really do any harm since the
interrupts are enabled only once in the power up and disabled before
power down.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>