drm/vmwgfx: integer underflow in vmw_cmd_dx_set_shader() leading to an invalid read

commit 5ed7f4b5 upstream.

If SVGA_3D_CMD_DX_SET_SHADER is called with a shader ID
of SVGA3D_INVALID_ID, and a shader type of
SVGA3D_SHADERTYPE_INVALID, the calculated binding.shader_slot
will be 4294967295, leading to an out-of-bounds read in vmw_binding_loc()
when the offset is calculated.

Cc: <stable@vger.kernel.org>
Fixes: d80efd5c ("drm/vmwgfx: Initial DX support")
Signed-off-by: Murray McAllister <murray.mcallister@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>