drm/i915/dp: Fix I2C/EDID handling with active DisplayPort to DVI converter

The DisplayPort standard (1.1a) states that:
  The I2C-over-AUX Reply field is valid only when Native AUX CH Reply
  field is AUX_ACK (00). When Native AUX CH Reply field is not 00, then,
  I2C-over-AUX Reply field must be 00 and be ignored.

This fixes broken EDID reading when using an active DisplayPort to
duallink DVI converter.  If the AUX CH replier chooses to defer the
transaction, a short read occurs and erroneous data is returned as
the i2c reply due to a lack of length checking and failure to check
for AUX ACK.

As a result, broken EDIDs can look like:
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: bc bc bc ff bc bc bc ff bc bc bc ac bc bc bc 45    ???.???.???????E
10: bc bc bc 10 bc bc bc 34 bc bc bc ee bc bc bc 4c    ???????4???????L
20: bc bc bc 50 bc bc bc 00 bc bc bc 40 bc bc bc 00    ???P???.???@???.
30: bc bc bc 01 bc bc bc 01 bc bc bc a0 bc bc bc 40    ???????????????@
40: bc bc bc 00 bc bc bc 00 bc bc bc 00 bc bc bc 55    ???.???.???.???U
50: bc bc bc 35 bc bc bc 31 bc bc bc 20 bc bc bc fc    ???5???1??? ????
60: bc bc bc 4c bc bc bc 34 bc bc bc 46 bc bc bc 00    ???L???4???F???.
70: bc bc bc 38 bc bc bc 11 bc bc bc 20 bc bc bc 20    ???8??????? ???
80: bc bc bc ff bc bc bc ff bc bc bc ff bc bc bc ff    ???.???.???.???.
...

which can lead to:
[drm:drm_edid_block_valid] *ERROR* EDID checksum is invalid, remainder
[drm:drm_edid_block_valid] *ERROR* Raw EDID:
<3>30 30 30 30 30 30 30 32 38 32 30 32 63 63 31 61  000000028202cc1a
<3>28 00 02 8c 00 00 00 00 18 00 00 00 00 00 00 00  (...............
<3>20 4c 61 73 74 20 62 65 61 63 6f 6e 3a 20 33 32   Last beacon: 32
<3>32 30 6d 73 20 61 67 6f 46 00 05 8c 00 00 00 00  20ms agoF.......
<3>36 00 00 00 00 00 00 00 00 0c 57 69 2d 46 69 20  6.........Wi-Fi
<3>52 6f 75 74 65 72 01 08 82 84 8b 96 24 30 48 6c  Router......$0Hl
<3>03 01 01 06 02 00 00 2a 01 00 2f 01 00 32 04 0c  .......*../..2..
<3>12 18 60 dd 09 00 10 18 02 00 00 01 00 00 18 00  ..`.............

Signed-off-by: David Flynn <davidf@rd.bbc.co.uk>
[ickle: fix up some surrounding checkpatch warnings]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org