drm/vc4: hdmi: Fix NULL vs IS_ERR() checks in vc5_hdmi_init_resources()

The devm_ioremap() function never returns error pointers, it returns
NULL.

Fixes: 83239891 ("drm/vc4: hdmi: Support the BCM2711 HDMI controllers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910100825.GC79916@mwanda