drm: bridge/dw_hdmi: avoid enabling interface in mode_set

On a mode set, DRM makes the following sequence of calls:
* for_each_encoder
*   bridge	mode_fixup
*   encoder	mode_fixup
* crtc		mode_fixup
* for_each_encoder
*   bridge	disable
*   encoder	prepare
*   bridge	post_disable
* disable unused encoders
* crtc		prepare
* crtc		mode_set
* for_each_encoder
*   encoder	mode_set
*   bridge	mode_set
* crtc		commit
* for_each_encoder
*   bridge	pre_enable
*   encoder	commit
*   bridge	enable

dw_hdmi enables the HDMI output in both the bridge mode_set() and also
the bridge enable() step.  This is duplicated work - we can avoid the
setup in mode_set() and just do it in the enable() stage.  This
simplifies the code a little.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>