drm/amdgpu/amdkfd: Avoid a couple hundred -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declarations to the end of the corresponding
structures. Notice that `struct dev_pagemap` is a flexible structure,
this is a structure that contains a flexible-array member.

struct dev_pagemap always has room for at least one range. amdgpu only
uses a single range. Therefore no change are needed to the allocation
of struct amdgpu_device.

Fix 283 of the following type of warnings:
    283 drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.h:111:28: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>