msm: mdss: Remove the unused has_buf pipe member

The has_buf was introduced for a special case where buffers may not
be queued to MDP directly. Remove the has_buf variable since it is
not used anymore.

Change-Id: Ie8137991dc771b8b17da1f35d4bbbeb62ae8658b
Signed-off-by: Saurabh Shah <saurshah@codeaurora.org>