drm/tidss: dispc: Define field masks being used

Now that we have all the accessors taking masks, we can create defines
for them and reuse them as needed.

It makes the driver easier to read, less prone to consistency issues,
and allows to reuse defines when needed.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250827-drm-tidss-field-api-v3-14-7689b664cc63@kernel.org


Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>