linuxfb/drm: Pick up the format from the config file

Like eglfs, linuxfb (when QT_QPA_FB_DRM=1) can be configured to
use a buffer and fb of a format other than the default XRGB8888.

Most notably, adding "mode": "rgb565" to the first output in the
config file pointed to by QT_QPA_KMS_CONFIG will switch over
to using 16 bpp dumb buffers and a QImage::Format_RGB16 wrapping
QImage. Note that linuxfb/drm has no multiple output support atm,
so only the first output is taken into account.

The BGR variants (e.g. xbgr8888) are available but cause no
difference when it comes to Qt's painting (still maps to RGB32 etc.).
This may need to be revisited later.

Has no effect on the regular (fbdev) path in linuxfb.

Task-number: QTBUG-63272
Change-Id: Ie7d0b05e3449b336104332d9568dab60b4bedaa8
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

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