drm/panfrost: Use GPU_MMU_FEATURES_VA_BITS/PA_BITS macros

As done in panthor, define and use these GPU_MMU_FEATURES_* macros,
which makes code easier to read and reuse.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250324185801.168664-3-ariel.dalessandro@collabora.com