drm/msm/sde: prevent clearing multirect flags during validate failure

In corner case where crtc->planemask and atomic state->planes[]
do not match, driver clears the multirect_index/multirect_mode
in the plane->state during validate failure case. This can lead
to multirect_index/multirect_mode mismatch for next commits during
failure condition.

Change-Id: Idccaa79431469e185ca46c1e40b040427d02a96d
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>