gallium: add PIPE_CAP_INVALIDATE_BUFFER

gallium: add PIPE_CAP_INVALIDATE_BUFFER

It makes sense to re-use pipe->invalidate_resource for the purpose of
glInvalidateBufferData, but this function is already implemented in vc4
where it doesn't have the expected behavior. So add a capability flag
to indicate that the driver supports the expected behavior.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>