drm/imagination: Break an object reference loop

In the Linux kernel, the following vulnerability has been resolved:

drm/imagination: Break an object reference loop

When remaining resources are being cleaned up on driver close,
outstanding VM mappings may result in resources being leaked, due
to an object reference loop, as shown below, with each object (or
set of objects) referencing the object below it:

PVR GEM Object
GPU scheduler "finished" fence
GPU scheduler “scheduled” fence
PVR driver “done” fence
PVR Context
PVR VM Context
PVR VM Mappings
PVR GEM Object

The reference that the PVR VM Context has on the VM mappings is a
soft one, in the sense that the freeing of outstanding VM mappings
is done as part of VM context destruction; no reference counts are
involved, as is the case for all the other references in the loop.

To break the reference loop during cleanup, free the outstanding
VM mappings before destroying the PVR Context associated with the
VM context.