drm/amd/display: set variable custom_backlight_curve0 storage-class-specifier to static

smatch reports
drivers/gpu/drm/amd/amdgpu/../display/modules/power/power_helpers.c:119:31:
  warning: symbol 'custom_backlight_curve0' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>