drm/dp: Add counters in the drm_dp_aux struct for I2C NACKs and DEFERs

These counters are used for Displayort compliance testing to detect error
conditions when executing tests 4.2.2.4 and 4.2.2.5 in the Displayport Link
CTS specificaiton. They determine whether to use the preferred/requested
mode or the failsafe mode during these tests.

V2:
- Addressed previous review feedback
- Updated commit message
- Changed from uint8_t to uint32_t

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Todd Previte <tprevite@gmail.com>
[danvet: s/uint32_t/unsigned/ for clearer intent. Also drop the i915
from the subject, it's all core stuff.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>