drm/i915/dsi: transmit brightness command in HS state

In Video Mode, if DSI transcoder is set to transmit packets
in LP Escape mode, screen flickering would be obseved when
brightness commands are continuously and quickly transmitted
to a panel.

The problem may be resolved by changing the mode to transmit
packets from Low Power to HS.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Lee Shawn C <shawn.c.lee@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Signed-off-by: William Tseng <william.tseng@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110010217.26759-1-william.tseng@intel.com