D3D12: fix D3D12_VERTEX_BUFFER_VIEW stride when both vertex layout handles are valid

[falia18]

Fix for this issue: https://github.com/falia18/bgfx/tree/pr-example/fix-d3d12-layout-issue
To repro launch 01-cubes with --d3d12, it gives this:

The fix makes the layout selection consistant (when both layouts are valid) between these lines:

bgfx/src/renderer_d3d12.cpp

Line 4256 in a9f6104

const uint16_t layoutIdx = !isValid(vb.m_layoutHandle) ? stream.m_layoutHandle.idx : vb.m_layoutHandle.idx;

bgfx/src/renderer_d3d12.cpp

Lines 6893 to 6895 in a9f6104

	const uint16_t layoutIdx = isValid(draw.m_stream[idx].m_layoutHandle)
	? draw.m_stream[idx].m_layoutHandle.idx
	: vb.m_layoutHandle.idx;

(It's my first PR sorry if I messed up something)