drm/i915: Fix kbuild test robot build error

has_transcoder() was unused because function which was using it,
intel_display_capture_error_state() defined under
CONFIG_DRM_I915_CAPTURE_ERROR.
Moving has_transcoder() to under CONFIG_DRM_I915_CAPTURE_ERROR.
No functional change.

Fixes: d54c1a51 ("drm/i915: Fix broken transcoder err state")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227175147.11362-1-anshuman.gupta@intel.com