drm/amd/display/dc/dce/dce_dmcu: Move 'abm_gain_stepsize' to only source file it's used in

And only declare it if it's to be used.

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_dmcu.h:320:23: warning: ‘abm_gain_stepsize’ defined but not used [-Wunused-const-variable=]

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: Mauro Rossi <issor.oruam@gmail.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>