msm: mdss: clear the pipe's dirty flag after sspp configuration

In the current implementation, pipe's dirty flag is not cleared when
there is pipe reuse. This change clears the pipe's dirty flag if the
pipe is being reused with no configuration changes but only buffer
handle.

CRs-Fixed: 884773
Change-Id: I8d62e944110cd887f341dd8b33a642a679473ff7
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>