Fix the brightness applet sliders being unable to change brightness.

powerdevil-6.7.5-patchset-1 cherry-picks upstream 57f190459
("applets/brightness: fix Slider binding issues", KDE-bug #523281), which
rewrites the two onMoved handlers to qualify the control objects as
`dialog.screenBrightnessControl` and `dialog.keyboardBrightnessControl`.

Upstream can do that because master also declares those as required
properties on PopupDialog and passes them from main.qml. That companion
change is not part of the patchset, so in 6.7.5 `dialog` (id of the
PlasmaExtras.Representation) has no such property: the expression evaluates
to undefined and every drag of the slider dies with

    PopupDialog.qml:82: TypeError: Cannot read property 'setBrightness' of undefined

leaving the display brightness unchanged while the slider snaps back.

Drop the `dialog.` qualifier so the handlers resolve the ids through the
inherited context, exactly as the surrounding code already does for
`model: screenBrightnessControl.displays` on line 49. The rest of the
patchset, which is what actually fixes the slider bindings, is kept.

Drop this patch once the tree moves to >=kde-plasma/powerdevil-6.7.6,
which carries the complete upstream change.

--- a/applets/brightness/PopupDialog.qml
+++ b/applets/brightness/PopupDialog.qml
@@ -79,7 +79,7 @@
 
                     stepSize: maxBrightness/100
 
-                    onMoved: value => dialog.screenBrightnessControl.setBrightness(displayName, value)
+                    onMoved: value => screenBrightnessControl.setBrightness(displayName, value)
                     onActiveFocusChanged: if (activeFocus) scrollView.positionViewAtItem(this)
                 }
 
@@ -132,7 +132,7 @@
                 KeyNavigation.backtab: KeyNavigation.up
                 KeyNavigation.tab: KeyNavigation.down
 
-                onMoved: value => dialog.keyboardBrightnessControl.brightness = value
+                onMoved: value => keyboardBrightnessControl.brightness = value
                 onActiveFocusChanged: if (activeFocus) scrollView.positionViewAtItem(this)
             }