Fix follow warning:
Checking drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c...
[drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:770]: (error) Invalid number
of character '{' when these macros are defined: ''.
Checking drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: CONFIG_ACPI...
[drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:770]: (error) Invalid number
of character '{' when these macros are defined: 'CONFIG_ACPI'.
......
Checking drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: CONFIG_X86...
[drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:770]: (error) Invalid number
of character '{' when these macros are defined: 'CONFIG_X86'.
Checking drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: _X86_...
[drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:770]: (error) Invalid number
of character '{' when these macros are defined: '_X86_'.
Checking drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: __linux__...
[drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:770]: (error) Invalid number
of character '{' when these macros are defined: '__linux__'.

Fixes: 97d798b2 ("drm/amdgpu: simplify ATIF backlight handling")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>