drm/vc4: Replace long HDMI udelay with usleep_range

Since we aren't in atomic context replace this long udelay with a
usleep_range.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1519475894-11701-1-git-send-email-stefan.wahren@i2se.com