drm/amd/display/dc/irq/irq_service: Make local function static

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/amd/amdgpu/../display/dc/irq/irq_service.c:82:31: warning: no previous prototype for ‘find_irq_source_info’ [-Wmissing-prototypes]

Cc: Harry Wentland <[e-mail]>
Cc: Leo Li <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Mauro Rossi <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/irq/irq_service.c

···
 	*irq_service = NULL;
 }
 
-const struct irq_source_info *find_irq_source_info(
+static const struct irq_source_info *find_irq_source_info(
 	struct irq_service *irq_service,
 	enum dc_irq_source source)
 {