drm/amdkfd: shrink bitmap size in struct svm_validate_context

A MAX_GPU_INSTANCE bits bitmap will suffice.

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