drm/i915: Check error return when converting pipe to connector

An error from intel_get_pipe_from_connector() would mean a bug somewhere
else, but we still should check for it to prevent some other more
obscure bug later.

v2:
- Fall back to a reasonable default instead of bailing out in case of
  error. (Jani)
v3:
- Fix s/PIPE_INVALID/INVALID_PIPE/ typo. (Jani)
v4:
- Fix bogus bracing around WARN() condition. (Ville)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-5-git-send-email-imre.deak@intel.com