drm/i915/psr: Tie PSR2 support to Y coordinate requirement

Although i915 don't implement aux sync frame through tests was
findout that pannels can do selective update when the y-coordinate
is also included in SDP, that is why it is required to run PSR2 in
i915.

So moving to only one place the sink requirements that the actual
driver needs to enable PSR2.

Also intel_psr2_config_valid() is called every time the crtc config
is computed, wasting some time every time it was checking for
Y coordinate requirement.

This allow us to nuke y_cord_support and some of VSC setup code that
was handling a scenario that would never happen(PSR2 without Y
coordinate).

Also here renaming intel_dp_get_y_cord_status() to
intel_dp_get_y_coord_required() as it more accurate to the name and
function of bit according to eDP spec.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328223046.16125-4-jose.souza@intel.com