drm/i915/dsi: Merge intel_dsi_disable/enable into their respective callers

intel_dsi_disable/enable only have one caller, merge them into their
respective callers.

Change msleep(2) into usleep_range(2000, 5000) to make checkpatch happy,
otherwise no functional changes.

The main advantage of this change is that it makes it easier to
follow all the steps of the panel enable / disable sequence when
reading the code.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/d7249612e6d2e9639ecd1d8d106ca37d5794f2a4.1488273823.git.jani.nikula@intel.com