drivers: display: st7735r: support ST7735S

ST7735S requires the COLMOD command be repeated as part of writing
pixels, otherwise it goes back to some kind of default which is likely
wrong.

ST7735S works correctly after this plus the previous two commits; tested
on STM32L462 + KD0096FM.

Signed-off-by: David Lamparter <equinox@diac24.net>