drm/mipi-dbi: Make bits per word configurable for pixel transfers

MIPI DCS write/set commands have 8 bit parameters except for the
write_memory commands where it depends on the pixel format.
drm_mipi_dbi does currently only support RGB565 which is 16-bit and it
has to make sure that the pixels enters the SPI bus in big endian format
since the MIPI DBI spec doesn't have support for little endian.

drm_mipi_dbi is optimized for DBI interface option 3 which means that the
16-bit bytes are swapped by the upper layer if the SPI bus does not
support 16 bits per word, signified by the swap_bytes member.

In order to support both 16-bit and 24-bit pixel transfers we need a way
to tell the DBI command layer the format of the buffer. Add a
write_memory_bpw member that the upper layer can use to tell how many
bits per word to use for the SPI transfer.

v4:
- Expand the commit message (Dmitry)

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-3-d7c2bcb9b78d@tronnes.org
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>

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