drm/tinydrm/mipi-dbi: Change reset active time

The MIPI DBI spec states that reset active/low time should be more
than 9us. Change from 20ms to 20us.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Tested-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110185940.53841-8-noralf@tronnes.org