drm/file: Add fdinfo helper for printing regions with prefix

This is motivated by the desire of some drivers (eg. Panthor) to print the
size of internal memory regions with a prefix that reflects the driver
name, as suggested in the previous documentation commit.

That means adding a new argument to print_size and making it available for
DRM users.

Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250130172851.941597-3-adrian.larumbe@collabora.com