fbdev/via: fix defined but not used warning

fbdev/via: fix defined but not used warning

BugLink: https://bugs.launchpad.net/bugs/1836117
[ Upstream commit b6566b47a67e07fdca44cf51abb14e2fbe17d3eb ]

Fix a build warning in viafbdev.c when CONFIG_PROC_FS is not enabled
by marking the unused function as __maybe_unused.

../drivers/video/fbdev/via/viafbdev.c:1471:12: warning: 'viafb_sup_odev_proc_show' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>