drm/amd/display: dumb_abm_lcd: avoid missing-prototype warnings

The dmub_abm_set_ambient_level() function has no caller and can
just be removed, the other ones have a declaration in the
header file and just need to see the prototype:

drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:122:14: error: no previous prototype for function 'dmub_abm_get_current_backlight' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:133:14: error: no previous prototype for function 'dmub_abm_get_target_backlight' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:144:6: error: no previous prototype for function 'dmub_abm_set_level' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:163:6: error: no previous prototype for function 'dmub_abm_set_ambient_level' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:183:6: error: no previous prototype for function 'dmub_abm_init_config' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:213:6: error: no previous prototype for function 'dmub_abm_set_pause' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:231:6: error: no previous prototype for function 'dmub_abm_set_pipe' [-Werror,-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_abm_lcd.c:251:6: error: no previous prototype for function 'dmub_abm_set_backlight_level' [-Werror,-Wmissing-prototypes]

Fixes: b8fe5637 ("drm/amd/display: Refactor ABM feature")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>