drm/tilcdc: Add tilcdc_write_mask() to tilcdc_regs.h

Add tilcdc_write_mask() for handling register field wider than one bit
and mask values for those fields.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>