drm/msm/sde: fix incorrect mixer height seen in dump

Panel height is seen as default for mixer height in dumps
even when destination scaler is enabled. Fix given to
reflect the correct mixer height.

Change-Id: I550ba71de2b27844fd6c948cbfa9f1ca0db9cd64
Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>