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

The drm_prime_pages_to_sg() function takes an 'unsigned int' argument to
store the length of the page vector. The size of the object in number of
CPU pages can not be negative, hence, use 'unsigned' variable to store
the number of pages, instead of the 'signed' one.

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>