rdp-gfx-surface: Delete GFX surface before clearing its members

When clearing a GFX surface, the graphics pipeline currently first
clears its members, which are, if they exist, the render surface and
the frame controller, and then deletes the GFX surface itself.
This has currently the effect, that when the client sends a frame
acknowledgement for a frame, that was rendered on a GFX surface, that
is about to be cleared, that the respective frame controller is used,
while it is in the process of being deleted leading to a crash, where a
NULL pointer is dereferenced.
The reason for this situation is that for each surface, a surface
context is created, which also contains a pointer to the GFX surface.
This is necessary to be able to map frame ids to surfaces at any time,
even when the respective surface is already deleted.
When clearing a surface, it is first removed from the surface table,
while the GFX mutex is locked, and then actually deleted, where the GFX
mutex has to be unlocked first, to not lock it again.
In this process of unlocking the GFX mutex and locking it again a frame
acknowledgement can happen, and in this situation, the frame controller
might be in the process of deletion, leading to a NULL pointer
dereference, where the frame controller or one of its members is being
looked up.

To fix this issue, simply delete the GFX surface first. This also
clears the pointer to the GFX surface in the surface context.
The other members of the GFX surface can afterwards be cleared without
any problems.

https://errors.ubuntu.com/problem/ecf34bd0b58c72cf2d1f03ba4222b71cc62e0d83