drivers: mipi_dbi: add mipi_dbi_release API

Some SPI based displays expect the ability to lock the SPI bus after a
transaction completes, or to hold CS low. In order to accommodate this
within the MIPI DBI layer, add the mipi_dbi_release API, which allows
SPI displays to hold then release the SPI bus used by the MIPI
abstraction layer.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>