This is only used by mipi-dbi drivers so move it there.

The reason this isn't moved to the SPI subsystem is that it will in a
later patch pass a dummy rx buffer for SPI controllers that need this.
Low memory boards (64MB) can run into a problem allocating such a "large"
contiguous buffer on every transfer after a long up time.
This leaves a very specific use case, so we'll keep the function here.
mipi-dbi will first go through a refactoring though, before this will
be done.

Remove SPI todo entry now that we're done with the tinydrm.ko SPI code.

v2: Drop moving the mipi_dbi_spi_init() declaration (Sam)

Cc: David Lechner <david@lechnology.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: : David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719155916.62465-8-noralf@tronnes.org