drm/i915: Generalize .set_signal_levels()

Currently .set_signal_levels() is only used by encoders in DP mode.
For most modern platforms there is no essential difference between
DP and HDMI, and both codepaths just end up calling the same function
under the hood. Let's get remove the need for that extra indirection
by moving .set_signal_levels() into the encoder from intel_dp.
Since we already plumb the crtc_state/etc. into .set_signal_levels()
the code will do the right thing for both DP and HDMI.

HSW/BDW/SKL are the only platforms that need a bit of care on
account of having to preload the hardware buf_trans register
with the full set of values. So we must still remember to call
hsw_prepare_{dp,hdmi}_ddi_buffers() to do said preloading, and
.set_signal_levels() will just end up selecting the correct entry
for DP, and also setting up the iboost magic for both DP and HDMI.

Note that previously on HSW/BDW/SKL we did write to DDI_BUF_CTL to
select the correct entry until link training started, now that we
call .set_signal_levels() already from hsw_ddi_pre_enable_dp() that
is no longer the case. But it's all safe now that the
intel_ddi_init_dp_buf_reg() call was hoisted up and it no longer
sets up the DDI_BUF_CTL_ENABLE bit (that is still deferred until
link training).

v2: Rebase due to has_{iboost,buf_trans_select}()
    Add some notes about the DDI_BUF_CTL situation on HSW/BDW/SKL (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-4-ville.syrjala@linux.intel.com