drm: move the buddy allocator from i915 into common drm

Move the base i915 buddy allocator code into drm
- Move i915_buddy.h to include/drm
- Move i915_buddy.c to drm root folder
- Rename "i915" string with "drm" string wherever applicable
- Rename "I915" string with "DRM" string wherever applicable
- Fix header file dependencies
- Fix alignment issues
- add Makefile support for drm buddy
- export functions and write kerneldoc description
- Remove i915 selftest config check condition as buddy selftest
  will be moved to drm selftest folder

cleanup i915 buddy references in i915 driver module
and replace with drm buddy

v2:
  - include header file in alphabetical order (Thomas)
  - merged changes listed in the body section into a single patch
    to keep the build intact (Christian, Jani)

Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>

drivers/gpu/drm/Makefile

@@ -18,7 +18,7 @@ drm-y       :=	drm_aperture.o drm_auth.o drm_cache.o \
 		drm_dumb_buffers.o drm_mode_config.o drm_vblank.o \
 		drm_syncobj.o drm_lease.o drm_writeback.o drm_client.o \
 		drm_client_modeset.o drm_atomic_uapi.o drm_hdcp.o \
-		drm_managed.o drm_vblank_work.o
+		drm_managed.o drm_vblank_work.o drm_buddy.o
 
 drm-$(CONFIG_DRM_LEGACY) += drm_agpsupport.o drm_bufs.o drm_context.o drm_dma.o \
 			    drm_legacy_misc.o drm_lock.o drm_memory.o drm_scatter.o \