drm/i915/gt: Prevent allocation on a banned context

If a context is banned even before we submit our first request to it,
report the failure before we attempt to allocate any resources for the
context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303080546.1140508-2-chris@chris-wilson.co.uk