drm/i915: Avoid a full port detection in the first eDP short pulse

Some eDP panels do not set a valid sink count value and even for the
ones that sets is should always be one for eDP, that is why it is not
cached in intel_edp_init_dpcd().

But intel_dp_short_pulse() compares the old count with the read one
if there is a mistmatch a full port detection will be executed, what
was happening in the first short pulse interruption of eDP panels
that sets sink count.

Instead of just skip the compasison for eDP panels, lets not read
the sink count at all for eDP.

v2: the previous version of this patch it was caching the sink count
in intel_edp_init_dpcd() but I was pointed out by Ville a patch that
handled a case of a eDP panel that do not set sink count and as sink
count is not used to eDP certification was choosed to just not read
it at all.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@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/20181121225441.18785-1-jose.souza@intel.com