The MIPI DBI standard support more pixel formats than what this helper
supports. Add an init function that lets the driver use different
format(s). This avoids open coding mipi_dbi_init() in st7586.

st7586 sets preferred_depth but this is not necessary since it only
supports one format.

v2: Forgot to remove the mipi->rotation assignment in st7586,
    mipi_dbi_init_with_formats() handles it.

Cc: David Lechner <david@lechnology.com>
Acked-by: David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719155916.62465-11-noralf@tronnes.org