drm/amdgpu: Remove UML build exclusion from Kconfig

drm/amdgpu: Remove UML build exclusion from Kconfig

The depends on !UML was added in commit dffe68131707 ("amdgpu: Avoid
building on UML") to work around build failures with allyesconfig on
UML. The original errors were:

 - smu7_hwmgr.c: incompatible pointer type 'struct cpuinfo_um *' vs
   'struct cpuinfo_x86 *' in intel_core_rkl_chk()
 - kfd_topology.c: 'struct cpuinfo_um' has no member named 'apicid'

Both issues have since been resolved independently:
 - intel_core_rkl_chk() has been removed entirely.
 - kfd_topology.c now uses a proper #ifdef CONFIG_X86_64 guard.
 - All other cpuinfo_x86/cpu_data() references in the driver are
   guarded by #if IS_ENABLED(CONFIG_X86) or #ifdef CONFIG_X86_64.

Removing this exclusion allows CONFIG_DRM_AMDGPU to be selected on UML,
which in turn enables running KUnit tests (such as amdgpu_dm_crc_test)
under UML without needing a full hardware-capable kernel build.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>