msm: sde: return success if no callback function in r1 ctl

Return success for sde_mdp_display_wait4comp if no callback
is setup in order to avoid returning non-zero error code.

CRs-Fixed: 1097957
Change-Id: Ic1fb20052a293878df09cd4f23cf3abe324cfbd3
Signed-off-by: Alan Kwong <akwong@codeaurora.org>