drm/amd/display: Turn global functions into static functions

Turn previously global functions into static functions to avoid
-Wmissing-prototype warnings, such as:

drivers/gpu/drm/amd/amdgpu/../display/dc/irq/dcn30/irq_service_dcn30.c:50:20:
warning: no previous prototype for function 'to_dal_irq_source_dcn30'
[-Wmissing-prototypes]
enum dc_irq_source to_dal_irq_source_dcn30(
                   ^
drivers/gpu/drm/amd/amdgpu/../display/dc/irq/dcn30/irq_service_dcn30.c:50:1:
note: declare 'static' if the function is not intended to be used outside
of this translation unit
enum dc_irq_source to_dal_irq_source_dcn30(
^
static
1 warning generated.

drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn316/dcn316_clk_mgr.c:488:6:
warning: no previous prototype for function
'dcn316_clk_mgr_helper_populate_bw_params' [-Wmissing-prototypes]
void dcn316_clk_mgr_helper_populate_bw_params(
     ^
drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn316/dcn316_clk_mgr.c:488:1:
note: declare 'static' if the function is not intended to be used outside
of this translation unit
void dcn316_clk_mgr_helper_populate_bw_params(
^
static
1 warning generated.

v2: drop is_timing_changed hunk (Alex)

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>