drm/fb-helper: Add fill_info() functions

drm/fb-helper: Add fill_info() functions

The fbdev split between fix and var information is kinda
pointless for drm drivers since everything is fixed: The fbdev
emulation doesn't support changing modes at all.

Create a new simplified helper and use it in the generic fbdev
helper code. Follow-up patches will beef it up more and roll
it out to all drivers.

v2: We need to keep sizes, since they might not match the fb dimensions
(Noralf)

v3: Fix typo in commit message and remove extraneous line in kerneldoc (Noralf)

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-1-daniel.vetter@ffwll.ch