drm/msm/sde: avoid configuring PE for virtual planes

Pixel extension of a pipe must be configured as per the RECT0
source size values even if the pipe is configured in multirect
mode. This change avoids virtual pipe overwriting the PE values
with RECT1 source size.

Change-Id: I006e9cc79a310d51f5d76bc9209e04d1ffd0884f
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>