[blitter] Initialize m_devicePixelRatio to fix crash

When using directfb platform the drawing of some widgets (e.g. Toolbar)
would crash the application.

Pixmap device pixel ratio can be used in QCommonStyle::drawControl() as
a denominator to calculate pixmap width or size. The drawing of some
widgets (e.g. Toolbar) would fail the assertion of a non-zero
denominator.

This commit initializes the device pixel ratio when generating
BlitterClass pixmaps and makes QBlittablePlatformPixmap::metric(
PdmDevicePixelRatio) return devicePixelRatio() instead of 1.

Task-number: QTBUG-38709
Change-Id: Ia8b96c5341b5ac1297c483deed1c695846398b88
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>