[drm/sde] fix uninitialized src/dst size in sde plane

SDE plane src/dst size is uninitialized and programmed to hw when
only color format is changed. This change will only configure hw
when src/dst size is calculated.

Change-Id: I5953f899a2c503b1d8f2577c28a67711b9ed9a67
Signed-off-by: Camus Wong <camusw@codeaurora.org>
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>