drm/amd/pm: mark irq functions as 'static'

Two newly added functions cause a warning because they lack a prototype:

drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0_6_ppt.c:1328:5: error: no previous prototype for 'smu_v13_0_6_set_irq_state' [-Werror=missing-prototypes]
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0_6_ppt.c:1368:5: error: no previous prototype for 'smu_v13_0_6_register_irq_handler' [-Werror=missing-prototypes]

They are only used locally, so just mark them static.

Fixes: 676915e4 ("drm/amd/pm: Add ih for SMU v13.0.6 thermal throttling")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>