dri: Fix robust context creation via EGL attribute

dri: Fix robust context creation via EGL attribute

driCreateContextAttribs() emits an error if bit
__DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS is set for an ES context.  But,
EGL_EXT_create_context_robustness and EGL 1.5 both allow creation of
robust ES contexts. One requests a robust ES context by setting the
EGL_CONTEXT_OPENGL_ROBUST_ACCESS *attribute*, which Mesa's EGL layer
translates into the __DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS *bit*.

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