drm/i915/dp: Limit link training clock recovery loop

drm/i915/dp: Limit link training clock recovery loop

Limit the link training clock recovery loop to 10 attempts at
LANEx_CR_DONE per DP 1.4 spec section 3.5.1.2.2 and 80 attempts for
pre-DP 1.4 (4 voltage levels x 4 preemphasis levels x
x 5 identical voltages tries). Some faulty USB-C MST hubs can
cause us to get stuck in this loop indefinitely requesting something
like:

    voltage swing: 0, pre-emphasis level: 2
    voltage swing: 1, pre-emphasis level: 2
    voltage swing: 0, pre-emphasis level: 3

over and over so max_vswing would never be reached,
drm_dp_clock_recovery_ok() would never return true and voltage_tries
would always get reset to 1. The driver sends those values to the hub
but the hub keeps requesting new values every time.

Changes in v2:
    - updated commit message (DK, Manasi)
    - defined DP_DP14_MAX_CR_TRIES (Marc)
    - made the loop iterate for max 10 times (Rodrigo, Marc)

Changes in v3:
    - changed error message to use DP_DP14_MAX_CR_TRIES

Changes in v4:
    - Updated the title to reflect the change
    - Updated the commit message
    - Added 80 attempts for pre-DP 1.4 devices

Changes in v5:
    - Removed DP_DP14_MAX_CR_TRIES from drm

v6: Updated comment to match kernel style (Rodrigo)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Marc Herbert <marc.herbert@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu@linux.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/20180720214413.29506-1-rodrigo.vivi@intel.com