drm/i915: implement vmap/vunmap GEM object functions

Implement i915_gem_vmap_object() and i915_gem_vunmap_object(),
based on i915_gem_dmabuf_vmap() and i915_gem_dmabuf_vunmap().

This enables a drm_client to use drm_client_buffer_vmap() and
drm_client_buffer_vunmap() on hardware using the i915 driver.

Tested with a currently out of tree pixelflut drm_client[1] on:
- Lenovo ThinkCentre M720q (CoffeeLake-S GT2 / Intel UHD Graphics 630)
- Dell Wyse N06D - 3030 LT (ValleyView on Intel Celeron N2807 SOC)

[1] XDP->DRM pixelflut: https://labitat.dk/wiki/Pixelflut-XDR



Signed-off-by: Asbjørn Sloth Tønnesen <asbjorn@asbjorn.st>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240629182513.78026-1-asbjorn@asbjorn.st