drm: test: Fix 32-bit issue in drm_buddy_test

The drm_buddy_test KUnit tests verify that returned blocks have sizes
which are powers of two using is_power_of_2(). However, is_power_of_2()
operations on a 'long', but the block size is a u64. So on systems where
long is 32-bit, this can sometimes fail even on correctly sized blocks.

This only reproduces randomly, as the parameters passed to the buddy
allocator in this test are random. The seed 0xb2e06022 reproduced it
fine here.

For now, just hardcode an is_power_of_2() implementation using
x & (x - 1).

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Arunpravin Paneer Selvam <arunpravin.paneerselvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230329065532.2122295-2-davidgow@google.com
Signed-off-by: Christian König <christian.koenig@amd.com>