drm/etnaviv: Clean up etnaviv_gem_get_pages

As in the etnaviv_gem_get_pages() function, the point to the drm_device
has already been cached to the 'dev' local variable. We can use it
directly, While at it, using 'unsigned int' type to count the number of
pages. As the drm_prime_pages_to_sg() function takes an unsigned int type
for its third argument. No functional change.

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[lst: Reword subject to make more generic and match patch content]
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>