drm/i915/dp: Sanitize DPCD revision check in intel_dp_get_dsc_sink_cap()

Check only the eDP or the DP specific DPCD revision depending on the
sink type. Pass the corresponding revision to the function, which allows
getting the DSC caps of a branch device (in an MST topology, which has
its own DPCD and so DPCD revision).

While at it use DP_DPCD_REV_14 instead of open coding it and for clarity
add a separate function to read out the DSC capability on eDP.

v2:
- Use DP_DPCD_REV_14 instead of open coding it. (Stan)
- Check EDP_DCPD_REV/DPCD_REV in a clearer way. (Ville)
v3:
- Fix the read-out for eDP in intel_dp_detect().

Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Stanislav Lisovskiy <[email protected]> (v1)
Reviewed-by: Ville Syrjälä <[email protected]> (v2)
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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