drm/msm/dp: Add DP controllers for sc7280

The eDP controller on SC7280 is similar to the eDP/DP controllers
supported by the current driver implementation.

SC7280 supports one EDP and one DP controller which can operate
concurrently.

This change adds the support for eDP and DP controller on sc7280.

Signed-off-by: Sankeerth Billakanti <quic_sbillaka@quicinc.com>

changes in v3:
    - Split into patches according to function (Dmitry Baryshkov)
    - Use DP_CONTROLLER_1 for eDP controller intf (Dmitry Baryshkov)
    - Use DP_CONTROLLER_0 for sc7280-dp (Dmitry Baryshkov)
    - Add macro in drm_helper.h for checking ssc capability (Stephen Boyd)
    - Use existing macro to check assr capability (Stephen Boyd)
    - Add comment for HPD_INIT_SETUP delay (Stephen Boyd)

changes in v2:
    - Don't initialize variables to 0 (Stephen Boyd)
    - Use const for read-only dpcd (Stephen Boyd)
    - Remove zero pixel clock check (Stephen Boyd)
    - Sort compatible strings alphabetically (Stephen Boyd)
    - Use pwm_bl.c for backlight instead of gpio (Stephen Boyd)
    - Change return type for functions returning always 0 (Matthias Kaehlcke)
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/1635839325-401-3-git-send-email-quic_sbillaka@quicinc.com

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>