drm/tilcdc: Allocate register storage based on the actual number registers

Allocate suspend/resume register storage based on the actual number
registers the driver is aware of. The static allocation for register
storage had fallen behind badly.

Reported-by: Michael Bode <michael@bumbleB.de>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>