gpu/drm: ring_mirror_list --> pending_list

gpu/drm: ring_mirror_list --> pending_list

Rename "ring_mirror_list" to "pending_list",
to describe what something is, not what it does,
how it's used, or how the hardware implements it.

This also abstracts the actual hardware
implementation, i.e. how the low-level driver
communicates with the device it drives, ring, CAM,
etc., shouldn't be exposed to DRM.

The pending_list keeps jobs submitted, which are
out of our control. Usually this means they are
pending execution status in hardware, but the
latter definition is a more general (inclusive)
definition.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/405573/
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Christian König <christian.koenig@amd.com>