msm: mdss: ignore buffer mapping if it's solid fill layer

Ignore mdp_buffer mapping when MDP_SOLID_FILL_LAYER is detected.
The client marked this mdp_input_layer as SOLID_FILL layer
without input buffer associated with this layer. MDP hardware
SOLID_FILL feature will be utilized.

Change-Id: Ica48dc5b0be69ac42e2f442d6114781790a3c56a
Signed-off-by: Lei Zhou <leizhou@codeaurora.org>