drm/i915/edp: don't write to DP_LINK_BW_SET when using rate select

The eDP 1.5 spec adds a clarification for eDP 1.4x:

> For eDP v1.4x, if the Source device chooses the Main-Link rate by way
> of DPCD 00100h, the Sink device shall ignore DPCD 00115h[2:0].

We write 0 to DP_LINK_BW_SET (DPCD 100h) even when using
DP_LINK_RATE_SET (DPCD 114h). Stop doing that, as it can cause the panel
to ignore the rate set method.

Moreover, 0 is a reserved value for DP_LINK_BW_SET, and should not be
used.

v2: Improve the comments (Ville)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9081


Tested-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205180551.2476228-1-jani.nikula@intel.com