drm/i915/dp: Extract drm_dp_read_sink_count()

And of course, we'll also need to read the sink count from other drivers
as well if we're checking whether or not it's supported. So, let's
extract the code for this into another helper.

v2:
* Fix drm_dp_dpcd_readb() ret check
* Add back comment and move back sink_count assignment in intel_dp_get_dpcd()
v5:
* Change name from drm_dp_get_sink_count() to drm_dp_read_sink_count()
* Also, add "See also:" section to kdocs

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-17-lyude@redhat.com