ofwfb: fix boot on LE

Some framebuffer properties obtained from the device tree were not being
properly converted to host endian.
Replace OF_getprop calls by OF_getencprop where needed to fix this.

This fixes boot on PowerPC64 LE, when using ofwfb as the system console.

Reviewed by: bdragon
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27475