drm/i915: cope with large i2c transfers

The hardware, according to the specs, is limited to 256 byte transfers,
and current driver has no protections in case users attempt to do larger
transfers. The code will just stomp over status register and mayhem
ensues.

Let's split larger transfers into digestable chunks. Doing this allows
Atmel MXT driver on Pixel 1 function properly (it hasn't since commit
9d8dc3e5

 "Input: atmel_mxt_ts -
implement T44 message handling" which tries to consume multiple
touchscreen/touchpad reports in a single transaction).

Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>