drm/vc4: hdmi: Fix some NULL vs IS_ERR() bugs

The devm_platform_ioremap_resource_byname() function doesn't return NULL,
it returns error pointers.  Update the checking to match.

Fixes: b93f07cf ("drm/vc4: move to devm_platform_ioremap_resource() usage")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a952e2b4-d4b8-49ac-abd9-9967c50f4a80@stanley.mountain


Signed-off-by: Maxime Ripard <mripard@kernel.org>