drm/i915: Fix broken transcoder err state

Skip the transcoder whose pipe is disabled while
initializing transcoder error state in 3 non-contiguous
display pipe system.

v2:
- Don't skip EDP_TRANSCODER error state. [Ville]
- Use a helper has_transcoder(). [Ville]
v3:
- Removed DSI transcoder case from has_transcoder(),
  and few other cosmetic changes. [Ville]

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224124004.26712-4-anshuman.gupta@intel.com