drm/vc4: hdmi: Use drm_atomic_helper_reset_crtc()

drm/vc4: hdmi: Use drm_atomic_helper_reset_crtc()

The current code uses a the reset_pipe() local function to reset the
CRTC outputs.

drm_atomic_helper_reset_crtc() has been introduced recently and it
performs exact same operations.

In order to avoid code duplication, use the new helper instead of the
local function.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250210132620.42263-4-herve.codina@bootlin.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>