drm: Declare hashtable as legacy

The DRM hashtable code is only used by internal functions for legacy
UMS drivers. Move the implementation behind CONFIG_DRM_LEGACY and the
declarations into legacy header files. Unexport the symbols.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129094841.22499-4-tzimmermann@suse.de