drm/amd/display: change to_dal_irq_source_dnc32() storage class specifier to static

sparse reports
drivers/gpu/drm/amd/amdgpu/../display/dc/irq/dcn32/irq_service_dcn32.c:39:20: warning: symbol 'to_dal_irq_source_dcn32' was not declared. Should it be static?

to_dal_irq_source_dnc32() is only referenced in irq_service_dnc32.c, so change its
storage class specifier to static.

Fixes: 0efd4374 ("drm/amd/display: add dcn32 IRQ changes")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>