svga: fix buffer binding flags initialization

svga: fix buffer binding flags initialization

If a buffer is created/initialized with glNamedBufferData we will
have no target (GL_ARRAY_BUFFER, GL_UNIFORM_BUFFER, etc) so the
svga_buffer::bind_flags will be zero until we try to get the buffer
handle.

This patch initializes the svga_buffer::bind_flags field when it's
zero.

This fixes the Piglit arb_uniform_buffer_object-rendering-dsa test.

Note that there's still issues in this area that'll have to be
addressed in the future.  For example, creating a buffer object
as GL_UNIFORM_BUFFER and later using it as a vertex buffer will
fail.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>