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 <qingqing.zhuo@amd.com>
CC: Lewis Huang <Lewis.Huang@amd.com>
CC: Aric Cyr <Aric.Cyr@amd.com>
CC: Alexander Deucher <Alexander.Deucher@amd.com>
CC: Harry Wentland <hwentlan@amd.com>
CC: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
CC: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
CC: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>