video: simplefb: modernise DT parsing

simplefb was using old style FDT parsing which doesn't behave well in
combination with livetree. Update it to use ofnode instead and add a
missing null check for the "format" property.

Standardise the error logging while we're here.

Fixes: 971d7e64 ("video: simplefb")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>