drm/i915/intel_i2c: use INDEX cycles for i2c read transactions

It is very common for an i2c device to require a small 1 or 2 byte write
followed by a read.  For example, when reading from an i2c EEPROM it is
common to write and address, offset or index followed by a reading some
values.

The i915 gmbus controller provides a special "INDEX" cycle for performing
such a small write followed by a read.  The INDEX can be either one or two
bytes long.  The advantage of using such a cycle is that the CPU has
slightly less work to do once the read with INDEX cycle is started.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>