drm/i915: fix up the edp power well check

Now that we track the cpu transcoder we need accurately in the pipe
config we can finally fix up the transcoder check. With the current
code eDP on port D will be broken since we'd errornously cut the
power.

For reference see

commit 2124b72e
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Mar 22 14:07:23 2013 -0300

    drm/i915: don't disable the power well yet

v2:
- Kill the now outdated comment (Paulo)
- Add the missing crtc->base.enabled check and consolidate it (Paulo)
- Smash all checks together, looks neater that way.

v3: Kill the unused encoder variable.

Cc: Takashi Iwai <tiwai@suse.de>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>