drm/panel/raspberrypi-touchscreen: Use independent I2C actions with delay.

We now have the hardware I2C controller pinmuxed to the drive the
display I2C, but this controller does not support clock stretching.
The Atmel micro-controller in the panel requires clock stretching
to allow it to prepare any data to be read.

Split the rpi_touchscreen_i2c_read into two independent transactions with
a delay between them for the Atmel to prepare the data.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>