drm/etnaviv: Use 'unsigned' type to count the number of pages

The unpin_user_pages() function takes an 'unsigned long' argument to
store the number of userspace pages, and the struct drm_gem_object::size
is a size_t type. The number of pages can not be negative, hence, use
'unsigned' variable to count the number of pages.

Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>