drm/vc4: Fix HDMI mode validation

vc4_hdmi_encoder_mode_valid(struct drm_encoder *crtc,

			    const struct drm_display_mode *mode)

{

	/* HSM clock must be 108% of the pixel clock.  Additionally,

	 * the AXI clock needs to be at least 25% of pixel clock, but

	 * HSM ends up being the limiting factor.

	/*

	 * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must

	 * be faster than pixel clock, infinitesimally faster, tested in

	 * simulation. Otherwise, exact value is unimportant for HDMI

	 * operation." This conflicts with bcm2835's vc4 documentation, which

	 * states HSM's clock has to be at least 108% of the pixel clock.

	 *

	 * Real life tests reveal that vc4's firmware statement holds up, and

	 * users are able to use pixel clocks closer to HSM's, namely for

	 * 1920x1200@60Hz. So it was decided to have leave a 1% margin between

	 * both clocks. Which, for RPi0-3 implies a maximum pixel clock of

	 * 162MHz.

	 *

	 * Additionally, the AXI clock needs to be at least 25% of

	 * pixel clock, but HSM ends up being the limiting factor.

	 */

	if (mode->clock > HSM_CLOCK_FREQ / (1000 * 108 / 100))

	if (mode->clock > HSM_CLOCK_FREQ / (1000 * 101 / 100))

		return MODE_CLOCK_HIGH;

	return MODE_OK;