drm: Add drm_{crtc/encoder/connector}_mode_valid()

Add a new helper to call crtc->mode_valid, connector->mode_valid
and encoder->mode_valid callbacks.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Dave Airlie <airlied@linux.ie>

Changes v2->v3:
	- Move helpers to drm_probe_helper.c (Daniel)
	- Squeeze patches that introduce helpers into a single
	one (Daniel)

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
[danvet: Make it compile when CONFIG_DRM_DP_AUX_CHARDEV is selected.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/b55c8bd029da219ff04e39086025c115731a49b1.1495720737.git.joabreu@synopsys.com