egl: add helper to combine two u32 into one u64

egl: add helper to combine two u32 into one u64

Use a helper to avoid the common issues of upcasting after the right shift
(losing the upper bits) and shifting signed values (sign gets shifted too).

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>