drm/fb-helper: Allow var->x/yres(_virtual) < fb->width/height again

Otherwise this can also prevent modesets e.g. for switching VTs, when
multiple monitors with different native resolutions are connected.

The depths must match though, so keep the != test for that.

Also update the DRM_DEBUG output to be slightly more accurate, this
doesn't only affect requests from userspace.

Bugzilla: https://bugs.freedesktop.org/99841


Fixes: 865afb11 ("drm/fb-helper: reject any changes to the fbdev")
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323085326.20185-1-michel@daenzer.net