drm/fb-helper: Move damage blit code and its setup into separate routine

Introduce a separate function for the blit code and its vmap setup. Done
in preparation of additional changes. No functional changes are made.

v3:
	* Use drm_WARN_ONCE() with an error message to print warning
v2:
	* print a single warning if damage blitter fails

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-8-tzimmermann@suse.de