drm/i915/display/ehl: Limit eDP to HBR2

Recent update in documentation defeatured eDP HBR3 for EHL and JSL.

v2:
- Remove dead code in ehl_get_combo_buf_trans()

v3:
- Rebase

BSpec: 32247
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201005175447.93430-1-jose.souza@intel.com