drm/amdkfd: Remove usage of alloc(sizeof(struct...

See https://kernel.org/doc/html/latest/process/coding-style.html
under "14) Allocating Memory" for rationale behind removing the
x=alloc(sizeof(struct) style and using x=alloc(sizeof(*x) instead

Signed-off-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>