boards: shields: arduino_giga_display_shield: Disable display SMH.

This allows all samples to work out of the box without requiring
SMH initialization. If DCMI (video) is used simultaneously and
more video buffers are needed, the application should enable SMH
for both the video and display subsystems and initialize it.

Signed-off-by: Ibrahim Abdalkader <i.abdalkader@gmail.com>