drm/i915: Use the correct max source link rate for MST

[why]
Previously used value was not safe to provide the correct value, i.e. it
could be 0 if not not configured, leading to no MST on this platform.

[how]
Do not use the value from BIOS, but from the structure populated at
encoder initialization time.

Fixes: 98025a62 ("drm/dp_mst: Use Extended Base Receiver Capability DPCD space")
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[fixed open coded drm_dp_link_rate_to_bw_code()]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430214531.24565-2-nikola.cornij@amd.com