drm/tinydrm/mi0283qt: Remove ili9341.h

No need for a public header file for the command macros.
Just include the necessary ones in the driver.

Also use the MIPI_DCS_PIXEL_FMT_16BIT macro.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110185940.53841-3-noralf@tronnes.org