drm/mipi-dbi: Remove mipi_dbi_machine_little_endian()

drm/mipi-dbi: Remove mipi_dbi_machine_little_endian()

mipi_dbi_machine_little_endian() should really have been called
mipi_dbi_framebuffer_little_endian() because that's the function it
performs. When I added support for these SPI displays I thought that the
framebuffers on big endian machines were also big endian, but I have
later learned that this is not the case. There's a bit in the fourcc code
that controls this: DRM_FORMAT_BIG_ENDIAN.

Just remove the function to avoid confusion. We can add big endian support
later should the need arise and we have hardware to test on.

Instead of just amending the docs, expand it to explain the endianness
handling.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-2-d7c2bcb9b78d@tronnes.org
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>