drm: Remove the drm_get_unmapped_area() helper

This has been only used by the vmwgfx driver and vmwgfx over the last
year removed support for transparent hugepages on vram leaving
drm_get_unmapped_area completely unused.

There's no point in keeping unused code in core drm.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220425203152.1314211-2-zack@kde.org