drm/amdkfd: Fix user queue validation on Gfx7/8

To workaround queue full h/w issue on Gfx7/8, when application create
AQL queue, the ring buffer bo allocate size is queue_size/2 and
map queue_size ring buffer to GPU in 2 pieces using 2 attachments, each
attachment map size is queue_size/2, with same ring_bo backing memory.

For Gfx7/8, user queue buffer validation should use queue_size/2 to
verify ring_bo allocation and mapping size.

Fixes: 68e599db ("drm/amdkfd: Validate user queue buffers")
Suggested-by: Tomáš Trnka <trnka@scm.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>