drm/imagination: Use pvr_vm_context_get()

I missed this open-coded kref_get() while trying to debug a refcount
bug, so let's use the helper function here to avoid that waste of time
again in the future.

Fixes: ff5f643d ("drm/imagination: Add GEM and VM related code")
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8616641d-6005-4b25-bc0a-0b53985a0e08@imgtec.com


Signed-off-by: Matt Coster <matt.coster@imgtec.com>