drm/mipi-dbi: Remove dependency on GEM CMA helper library

The MIPI DBI helpers access struct drm_gem_cma_object.vaddr in a
few places. Replace all instances with the correct generic GEM
functions. Use drm_gem_fb_vmap() for mapping a framebuffer's GEM
objects and drm_gem_fb_vunmap() for unmapping them. This removes
the dependency on CMA helpers within MIPI DBI.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211106193509.17472-2-tzimmermann@suse.de