drm/i915/uc: perma-pin firmwares

Now that each FW has its own reserved area, we can keep them always
pinned and skip the pin/unpin dance on reset. This will make things
easier for the 2-step HuC authentication, which requires the FW to be
pinned in GGTT after the xfer is completed.
Since the vma is now valid for a long time and not just for the quick
pin-load-unpin dance, the name "dummy" is no longer appropriare and has
been replaced with vma_res. All the functions have also been updated to
operate on vma_res for consistency.
Given that we pin the vma behind the allocator's back (which is ok
because we do the pinning in an area that was previously reserved for
thus purpose), we do need to explicitly re-pin on resume because the
automated helper won't cover us.

v2: better comments and commit message, s/dummy/vma_res/

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531235415.1467475-2-daniele.ceraolospurio@intel.com