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If the SVM range has no GPU access nor access-in-place attribute,
validate and map to GPU should skip the range.

Add NULL pointer check if find_first_bit(ctx->bitmap, MAX_GPU_INSTANCE)
returns MAX_GPU_INSTANCE as gpuidx if ctx->bitmap is empty.
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Alex Sierra <alex.sierra@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>