package/qt5/qt5base: fix imx6 eglfs support with imx-gpu-viv

package/qt5/qt5base: fix imx6 eglfs support with imx-gpu-viv

Last commit fixed eglfs_kms support for i.MX8 platforms that required to
declare imx-gpu-viv as the gbm provider.
However, this broke the eglfs "fbdev" imx6 support as gbm isn't provided
in this case. So limit the gbm option to imx-gpu-viv when wayland
backend is used only.

Fixes: 82fb51d3b5 ("package/qt5/qt5base: allow to use imx-gpu-viv as GBM
provider")

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>