drm/format-helper: Rework format-helper conversion functions

Move destination-buffer clipping from all format-helper conversion
functions into callers. Support destination-buffer pitch. Only
distinguish between system and I/O memory, but use same logic
everywhere.

Simply harmonize the interface and semantics of the existing code.
Not all conversion helpers support all combinations of parameters.
We have to add additional features when we need them.

v2:
	* fix default destination pitch in drm_fb_xrgb8888_to_gray8()
	  (Noralf)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-5-tzimmermann@suse.de