drm/vkms: Add framebuffer and plane helpers

This patch appends the minimum helpers related to framebuffer and plane
to make vkms minimally usable.

Changes since V1:
- None
Changes since V2:
- Squash "Add plane helper struct" and "Add helper for framebuffer
  create"
Changes since V3:
 Daniel Vetter:
 - Remove atomic_check from plane helper

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/d37807da7d3b39444b4c9abb149fe3c518d07c61.1531402095.git.rodrigosiqueiramelo@gmail.com