drm/prime: Use unsigned type for number of pages

The number of pages can never be negative, so an unsigned type is
enough. This also matches the type of the n_pages argument of the
sg_alloc_table_from_pages() function.

Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --cc drivers/gpu/drm/drm_prime.c
Simple merge

diff --cc include/drm/drmP.h
Simple merge