drm/dbi: Support DBI typec1 read operations

Implement SPI reads for typec1, for SPI controllers that
can support 9bpw in addition to 8bpw (such as GPIO bit-banged
SPI).

9bpw emulation is not supported but we have to start with
something.

This is used by s6e63m0 to read display MTP information
which is used by the driver for backlight control.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210614181135.1124445-1-linus.walleij@linaro.org