OMAPDSS: Change struct reg_field to dispc_reg_field

Avoid colision with regmap's struct reg_field definition by renaming
omapdss's struct reg_field to dispc_reg_field, and moving it inside
dispc.c as that's the only place it is used.

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