drm/msm/sde: fix partial update concurrency check with CWB

In sde driver current validation of concurrent writeback with
the partial update rects on crtc, last committed partial-update
roi on CRTC is referred. This restricts usage of partial update
and disable CWB in single frame. This change fixes by referring
user programmed PU roi for current frame. This change also relaxes
modeset check in virtual encoder, to support PU with CWB disable
with out an additional frame in between.

Change-Id: Iaf87271a2c0671a271dd3253200a8dfe323f1fc9
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>