drm/tests/buddy: stop using PAGE_SIZE

Gives the wrong impression that min page-size has to be tied to the CPU
PAGE_SIZE.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240229105112.250077-4-matthew.auld@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>