board/freescale/imx6-sabresd: improve Qt5 Cinematic Demo instructions

QT_QPA_EGLFS_ALWAYS_SET_MODE=1 needs to be exported, otherwise the Qt5
Cinematic demo throws a storm of the following messages:

Could not queue DRM page flip on screen LVDS1 (Invalid argument)

Improve the instructions by mentioning such requirement.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit d9ccd8f0fc206805deca4ce6ae6b81480a1ead95)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/freescale/imx6-sabresd/readme.txt

@@ -80,6 +80,7 @@ Running kmscube application:
 Running Qt5 Cinematic Demo:
 
 # export QT_QPA_EGLFS_KMS_CONFIG=/root/sabresd.json
+# export QT_QPA_EGLFS_ALWAYS_SET_MODE=1
 # /usr/share/Qt5/CinematicExperience/Qt5_CinematicExperience
 
 Running gl2mark benchmark: