drm/amdkfd: fix stack size in svm_range_validate_and_map

Allocate large local variable on heap to avoid exceeding the
stack size:
drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_svm.c: In function ‘svm_range_validate_and_map’:
drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_svm.c:1690:1: warning: the frame size of 2360 bytes is larger than 2048 bytes [-Wframe-larger-than=]

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>