drm/tinydrm: remove call to mipi_dbi_init() from mipi_dbi_spi_init()

This removes the call to mipi_dbi_init() from mipi_dbi_spi_init() so that
drivers can have a driver-specific implementation if needed.

Suggested-by: Noralf Trønnes <noralf@tronnes.org>
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/1501799630-1650-2-git-send-email-david@lechnology.com