drm/i915/bxt: VSwing programming sequence

VSwing programming sequence as specified in the updated BXT BSpec

v2: Satheesh's review comments addressed.
- clear value before setting into registers
- move print statement to bxt function
Other changes
- since signal level will not be set into DDI_BUF_CTL, the value need
  not be returned to intel_dp_set_signal_levels(). Making the bxt
  specific function to return void and setting signal_levels = 0 for
  bxt inside intel_dp_set_signal_levels()
- instead of signal levels, printing vswing level and pre-emphasis
  level
- in case none of the pre-emphasis levels or vswing levels are set,
  setting default of 400mV + 0dB

v3: Satheesh's review comments
- Check for mask before printing signal_levels.
- Removing redundant register writes
- Call intel_prepare_ddi_buffers only for HAS_PCH_SPLIT
- Making register write part generic as it will be required for HDMI as
  well.

Re-structure the code to include an array for vswing related values, set
signal levels

v4: Satheesh's review comments
- Rebase over latest renaming patches
- use hsw_signal_levels for HAS_DDI
Other changes
- Modified vswing_sequence() func definition
- Rebased on top of register macro definitions

v5: Satheesh's review comments
- Check ddi translation table size

v6: Imre's review comments
- removed comments in vswing sequence
- added vswing, pre-emphasis prints in intel_dp_set_signal_levels
- added comment explaining use of DP vswing values for eDP
- initialize n_entries and ddi_transaltion table based on encoder type
- create bxt_ddi_buf_trans structure and use decimal values
- adding a flag in bxt buffer translation table to indicate def entry

v7: (imre)
- squash in Vandana's "VSwing register definition",
  "HDMI VSwing programming", "Re-enable vswing programming",
  "Fix vswing sequence" patches
- use BXT_PORT_* regs directly instead of via a temp var
- simplify BXT_PORT_* macro definitions
- add code comment why we read lane while write group registers
- fix readout of DP_TRAIN_PRE_EMPHASIS in debug message

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com> (v6)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>