drm/i915/dp: Correctly advertise HBR3 for GEN11+

intel_dp_set_source_rates() calls intel_dp_is_edp(), which is unsafe to
use before encoder_type is set. This caused GEN11+ to incorrectly strip
HBR3 from source rates for edp. Move intel_dp_set_source_rates() to
after encoder_type is set. Add comment to intel_dp_is_edp() describing
unsafe usages.

v2: Alter intel_dp_set_source_rates final position (Ville/Manasi).
    Remove outdated comment (Ville).
    Slight optimization of control flow in intel_dp_init_connector.
    Slight rewording in commit message.

Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630233310.10191-1-matthew.s.atwood@intel.com