drm/i915/dp: read sink count to a temporary variable first

Don't clobber intel_dp->sink_count with the raw value.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/37d3222115172922fcd5ab038238359935bd561f.1491485983.git.jani.nikula@intel.com