drm/buddy: Move internal helpers to buddy.c

drm/buddy: Move internal helpers to buddy.c

Move gpu_buddy_block_state(), gpu_buddy_block_is_allocated(),
and gpu_buddy_block_is_split() from gpu_buddy.h to gpu_buddy.c
as static functions since they have no external callers.

Remove gpu_get_buddy() as it was an unused exported wrapper
around the internal __get_buddy().

No functional changes.

v2:
- Rebased after DRM buddy allocator moved to drivers/gpu/
- Keep gpu_buddy_block_is_free() in header since it's now
  used by drm_buddy.c
- Updated commit message

Cc: Christian König <christian.koenig@amd.com>
Cc: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Sanjay Yadav <sanjay.kumar.yadav@intel.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patch.msgid.link/20260212092527.718455-6-sanjay.kumar.yadav@intel.com