drm/i915: Silence compiler warning

Silence the following -Wmaybe-uninitialized warnings and make the code
more clear.

drivers/gpu/drm/i915/intel_display.c: In function ‘__intel_set_mode’:
drivers/gpu/drm/i915/intel_display.c:11844:14: warning: ‘crtc_state’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return state->mode_changed || state->active_changed;
              ^
drivers/gpu/drm/i915/intel_display.c:11854:25: note: ‘crtc_state’ was declared here
  struct drm_crtc_state *crtc_state;
                         ^
drivers/gpu/drm/i915/intel_display.c:11868:6: warning: ‘crtc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (crtc != intel_encoder->base.crtc)
      ^
drivers/gpu/drm/i915/intel_display.c:11853:19: note: ‘crtc’ was declared here
  struct drm_crtc *crtc;

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>