drm/panfrost: Replace DRM driver allocation method with newer one

drm/panfrost: Replace DRM driver allocation method with newer one

Drop the deprecated DRM driver allocation method in favour of
devm_drm_dev_alloc(). Overall just make it the same as in Panthor.
Also discard now superfluous generic and platform device pointers inside
the main panfrost device structure.

Some ancient checkpatch issues unearthed as a result of these changes
were also fixed, like lines too long or double assignment in one line.

Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://lore.kernel.org/r/20251019145225.3621989-2-adrian.larumbe@collabora.com
Signed-off-by: Steven Price <steven.price@arm.com>