drm/dp: Correctly mask DP_TRAINING_AUX_RD_INTERVAL values for DP 1.4

DP_TRAINING_AUX_RD_INTERVAL with DP 1.3 spec changed bit scheeme from 8
bits to 7 in DPCD 0x000e. The 8th bit is used to identify extended
receiver capabilities. For panels that use this new feature wait interval
would be increased by 512 ms, when spec is max 16 ms. This behavior is
described in table 2-158 of DP 1.4 spec address 0000eh.

With the introduction of DP 1.4 spec main link clock recovery was
standardized to 100 us regardless of TRAINING_AUX_RD_INTERVAL value.

To avoid breaking panels that are not spec compiant we now warn on
invalid values.

V2: commit title/message, masking all 7 bits, warn on out of spec values.
V3: commit message, make link train clock recovery follow DP 1.4 spec.
V4: style changes
V5: typo
V6: print statement revisions, DP_REV to DPCD_REV, comment correction
V7: typo
V8: Style
V9: Strip out DPCD_REV_XX into seperate patch
v10: DPCD_REV_XX to DP_DPCD_REV_XX

Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180504221800.17830-2-matthew.s.atwood@intel.com