drm/tilcdc: Fix load mode bit-field setting in tilcdc_crtc_enable()

Set LCDC_PALETTE_LOAD_MODE bit-field with new tilcdc_write_mask()
instead of tilcdc_set(). Setting a bit-fields with tilcdc_set() is
fundamentally broken.

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