drm/sharp-memory: Do not access GEM-DMA vaddr directly

drm/sharp-memory: Do not access GEM-DMA vaddr directly

Use DRM's shadow-plane helper to map and access the GEM object's buffer
within kernel address space. Encapsulates the vmap logic in the GEM-DMA
helpers.

The sharp-memory driver currently reads the vaddr field from the GME
buffer object directly. This only works because GEM code 'automagically'
sets vaddr.

Shadow-plane helpers perform the same steps, but with correct abstraction
behind drm_gem_vmap(). The shadow-plane state provides the buffer address
in kernel address space and the format-conversion state.

v2:
- fix typo in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250627152327.8244-1-tzimmermann@suse.de