drm/i915: Replace kmalloc() + copy_from_user() with memdup_user()

drm/i915: Replace kmalloc() + copy_from_user() with memdup_user()

Replace kmalloc() followed by copy_from_user() with memdup_user() to
improve and simplify set_context_image(), and to silence the following
Coccinelle/coccicheck warning reported by memdup_user.cocci:

  WARNING opportunity for memdup_user

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20250902081046.35463-2-thorsten.blum@linux.dev