drm/tinydrm: export mipi_dbi_buf_copy and mipi_dbi_spi_cmd_max_speed

This exports the mipi_dbi_buf_copy() and mipi_dbi_spi_cmd_max_speed()
functions so that they can be shared with other drivers.

Signed-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1511122328-31133-4-git-send-email-david@lechnology.com