drm/simple-kms: Remove drm_gem_simple_display_pipe_prepare_fb()

The helper drm_gem_simple_display_pipe_prepare_fb() is simple-KMS'
default implementation for prepare_fb. Remove the call from drivers
that set it explicitly. Then inline the helper into the only caller
within simple-kms helpers. No functional changes.

Simple-KMS drivers that implement the prepare_fb callback should call
drm_gem_plane_helper_prepare_fb() directly.

v2:
	* fix typo in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202125644.7917-2-tzimmermann@suse.de