drm/msm: avoid double-attaching hdmi/edp bridges

Each of hdmi and edp are already attached in msm_*_bridge_init. A second
attachment returns -EBUSY, failing the driver load.

Tested with HDMI on IFC6410 (APQ8064 / MDP4), but eDP case should be
analogous.

Fixes: 3ef2f119 (drm/msm: Use drm_attach_bridge() to attach a bridge to an encoder)
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org> (hdmi part)
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>