drm/panfrost: Use u64 for size in lock_region

Mali virtual addresses are 48-bit. Use a u64 instead of size_t to ensure
we can express the "lock everything" condition as ~0ULL without
overflow. This code was silently broken on any platform where a size_t
is less than 48-bits; in particular, it was broken on 32-bit armv7
platforms which remain in use with panfrost. (Mainly RK3288)

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Suggested-by: Rob Herring <robh@kernel.org>
Tested-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210824173028.7528-3-alyssa.rosenzweig@collabora.com