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>

drivers/gpu/drm/amd/display/dc/dce/dce_dmcu.c

@@ -65,6 +65,10 @@
 //Register access policy version
 #define mmMP0_SMN_C2PMSG_91				0x1609B
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
+static const uint32_t abm_gain_stepsize = 0x0060;
+#endif
+
 static bool dce_dmcu_init(struct dmcu *dmcu)
 {
 	// Do nothing

drivers/gpu/drm/amd/display/dc/dce/dce_dmcu.h

@@ -317,6 +317,4 @@ struct dmcu *dcn21_dmcu_create(
 
 void dce_dmcu_destroy(struct dmcu **dmcu);
 
-static const uint32_t abm_gain_stepsize = 0x0060;
-
 #endif /* _DCE_ABM_H_ */