powerpc/powermac: Use of_property_match_string() in pmac_has_backlight_type()

Replace an of_get_property() call by of_property_match_string()
so that this function implementation can be simplified.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/linuxppc-dev/d9bdc1b6-ea7e-47aa-80aa-02ae649abf72@csgroup.eu/


Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/linuxppc-dev/87cyk97ufp.fsf@mail.lhotse/


Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/ede25e03-7a14-4787-ae1b-4fc9290add5a@web.de