drm/amd/display: fix compilation error on allmodconfig

when compiled with allmodconfig option, there are error
messages as below:

ERROR: modpost:
"mod_color_is_table_init"
[drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost:
"mod_color_get_table"
[drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost:
"mod_color_set_table_init_state"
[drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

To fix the issue, this commits removes
CONFIG_DRM_AMD_DC_DCN guard in color/makefile.

Signed-off-by: Qingqing Zhuo <[e-mail]>
CC: Lewis Huang <[e-mail]>
CC: Aric Cyr <[e-mail]>
CC: Alexander Deucher <[e-mail]>
CC: Harry Wentland <[e-mail]>
CC: Nicholas Kazlauskas <[e-mail]>
CC: Bhawanpreet Lakha <[e-mail]>
CC: Stephen Rothwell <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/modules/color/Makefile

···
 # Makefile for the color sub-module of DAL.
 #
 
-MOD_COLOR = color_gamma.o
-
-ifdef CONFIG_DRM_AMD_DC_DCN
-MOD_COLOR += color_table.o
-endif
+MOD_COLOR = color_gamma.o color_table.o
 
 AMD_DAL_MOD_COLOR = $(addprefix $(AMDDALPATH)/modules/color/,$(MOD_COLOR))
 #$(info ************  DAL COLOR MODULE MAKEFILE ************)