drm/fb-helper: move zeroing code to drm_fb_helper_fill_var

__fill_var is used by both drm_fb_helper_check_var and
drm_fb_helper_fill_var.  In drm_fb_helper_check_var, it is possible that
some of the variables in fb_info-> var which are currently being zero'ed
have pre-existing values.  Zeroing these causes some fb tests to fail
with (from the Xorg.log):

[     9.897] (II) Module fbdevhw: vendor="X.Org Foundation"
[     9.897]    compiled for 1.21.1.8, module version = 0.0.2
[     9.897]    ABI class: X.Org Video Driver, version 25.2
[     9.898] (II) FBDEV(0): using default device
[     9.901] (==) FBDEV(0): Depth 24, (==) framebuffer bpp 32
[     9.902] (==) FBDEV(0): RGB weight 888
[     9.902] (==) FBDEV(0): Default visual is TrueColor
[     9.902] (==) FBDEV(0): Using gamma correction (1.0, 1.0, 1.0)
[     9.902] (II) FBDEV(0): hardware: virtio_gpudrmfb (video memory:
4000kB)
[     9.902] (DB) xf86MergeOutputClassOptions unsupported bus type 0
[     9.903] (II) FBDEV(0): checking modes against framebuffer device...
[     9.904] (II) FBDEV(0):     mode "640x480" test failed
[     9.904] (II) FBDEV(0):     mode "640x480" test failed
[     9.904] (II) FBDEV(0):     mode "640x480" test failed
[     9.904] (II) FBDEV(0):     mode "640x480" test failed
[     9.904] (II) FBDEV(0):     mode "640x480" not found
[     9.904] (II) FBDEV(0): checking modes against monitor...
[     9.905] (II) FBDEV(0): Virtual size is 1280x800 (pitch 1280)
[     9.905] (**) FBDEV(0):  Built-in mode "current"
[     9.905] (==) FBDEV(0): DPI set to (96, 96)

Previously, these values were not modified.  Moving the zero'ing of the
variables to drm_fb_helper_fill_var resolves the issue.

Fixes: ee4cce0a ("drm/fb-helper: fix input validation gaps in check_var")
Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>