drm/format-helper: Add drm_fb_swab()

This replaces drm_fb_swab16() with drm_fb_swab() supporting 16 and 32-bit.
Also make pixel line caching optional.

v2:
- Bail out on cpp != 2 && 4 (Sam)

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200509141619.32970-8-noralf@tronnes.org