drm/sun4i: tcon: Rename Dithering related register macros

Dithering is only supported for TCON channel 0. Throughout the datasheet
all the names associated with these register are prefixed "TCON0",
instead of "TCON". The only exception is the control register
"TCON_FRM_CTL_REG".

Rename the macros to reflect this.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907041948.19913-3-wens@csie.org