drm/amd/display/modules/power/power_helpers: Staticify local functions

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

 drivers/gpu/drm/amd/amdgpu/../display/modules/power/power_helpers.c:281:6: warning: no previous prototype for ‘fill_iram_v_2’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/modules/power/power_helpers.c:455:6: warning: no previous prototype for ‘fill_iram_v_2_2’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/modules/power/power_helpers.c:601:6: warning: no previous prototype for ‘fill_iram_v_2_3’ [-Wmissing-prototypes]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/modules/power/power_helpers.c

@@ -278,7 +278,7 @@ static void fill_backlight_transform_table_v_2_2(struct dmcu_iram_parameters par
 	}
 }
 
-void fill_iram_v_2(struct iram_table_v_2 *ram_table, struct dmcu_iram_parameters params)
+static void fill_iram_v_2(struct iram_table_v_2 *ram_table, struct dmcu_iram_parameters params)
 {
 	unsigned int set = params.set;
 
@@ -452,7 +452,7 @@ void fill_iram_v_2(struct iram_table_v_2 *ram_table, struct dmcu_iram_parameters
 			params, ram_table);
 }
 
-void fill_iram_v_2_2(struct iram_table_v_2_2 *ram_table, struct dmcu_iram_parameters params)
+static void fill_iram_v_2_2(struct iram_table_v_2_2 *ram_table, struct dmcu_iram_parameters params)
 {
 	unsigned int set = params.set;
 
@@ -598,7 +598,7 @@ void fill_iram_v_2_2(struct iram_table_v_2_2 *ram_table, struct dmcu_iram_parame
 			params, ram_table, true);
 }
 
-void fill_iram_v_2_3(struct iram_table_v_2_2 *ram_table, struct dmcu_iram_parameters params, bool big_endian)
+static void fill_iram_v_2_3(struct iram_table_v_2_2 *ram_table, struct dmcu_iram_parameters params, bool big_endian)
 {
 	unsigned int i, j;
 	unsigned int set = params.set;