drm/i915: reset intel_encoder->type when DP or HDMI is detected

When intel_hdmi_detect detects a monitor, set intel_encoder->type with
INTEL_OUTPUT_HDMI. Same for DP.

This should not break the current code because these variables never
change. This will be used after we create the DDI encoder because it
will have both DP and HDMI connectors.

We won't support eDP+HDMI on the same port, so if an encoder is eDP we
should expect it to always remain eDP and never change.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>