drm/i915: Reset GMBUS controller after NAK

Once a NAK has been asserted by the slave, we need to reset the GMBUS
controller in order to continue. This is done by asserting the Software
Clear Interrupt bit and then clearing it again to restore operations.

If we don't clear the NAK, then all future GMBUS xfers will fail,
including DDC probes and EDID retrieval.

v2: Add some comments as suggested by Keith Packard.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35781


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: "Mengmeng Meng" <mengmeng.meng@intel.com>