msm: mdss: reset pixel extension flag if validation fails

While validating the pixel extension values, reset the pixel
extension flag in case of failure. If this flag is not reset,
driver will programm the wrong configuration into hardware
registers in the commit ioctl.

Change-Id: I6fb90ca331f39e704a8aa915a04b6be776bcff55
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>