drm/msm/sde: add dim layer support

Dim layer is a layer mixer feature added from SDE 4.0, where the
specified rect can be color filled in the LM output. It can be
configured for any stage, with the constraint that no other
pipes should be staged in that stage. Also, it would not work
along with src_split logic, so the dim layer should fall within
the LM boundaries.

Change-Id: I2d7f3d22ed51dbc8d8d8cffc7a12ed81f249ee9f
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>