drm_hwcomposer: check if the primary plane is being used by any crtc

Some DRM driver such as omap_drm allow sharing primary plane between CRTCs.
drm_hwcomposer isn't ready for such a scenarios and fails with an error:

    Found more than 1 primary plane for CRTC

Don't report multiple primary planes for single CRTC to satisfy the drm_hwc.

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>