Let the user know what went wrong in drm_gem_fb_init_with_funcs
failure paths.

v2: use proper format specifier for size_t (kernel test robot)
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ZS4eX0PtTuNvHezILcTWeWINOkRyMS4krXND3cIE@cp4-web-032.plabs.ch