drm/atomic-helper: Bump vblank timeout to 100 ms

The 50 ms default timeout waiting for vblanks is too small
for the first vblank from the ST-Ericsson MCDE display
controller over DSI. Presumably this is because the DSI
display is command-mode only and the state machine will
take some time setting up its state for the first display
update, and we hit a timeout. 100 ms makes it pass without
problems.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190430093746.26485-1-linus.walleij@linaro.org