drm/fb-cma-helper: Add drm_fb_cma_get_gem_addr()

Add function drm_fb_cma_get_gem_addr() which return the physical address
of framebuffer (1st pixel). This function will usually be called by plane
callback (atomic_update).

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1492164819-10513-3-git-send-email-yannick.fertre@st.com


Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Eric Anholt <eric@anholt.net>