via/via_irq: use __func__ to replace string function name

This change also fix checkpatch.pl warning:
WARNING: Prefer using '"%s...", __func__' to using
'via_driver_irq_postinstall', this function's name, in a string
+	DRM_DEBUG("via_driver_irq_postinstall\n");

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119072957.108941-1-bernard@vivo.com