disp: msm: sde: remove checks in LTM hist disable function

The right_mixer and hist_en checks in LTM hist disable function are
not valid for dual display use cases, since in dual display use cases,
right_mixer will always be NULL.
When device enters suspend state, hist_en will be set to false in
sde_cp_crtc_suspend(), and together with the right_mixer check,
it will block the ltm buffer lists from resetting and cause LTM has
no buffers to use after resume.

Change-Id: I51b1b00f4af59e42105322de68019479c65f2fbb
Signed-off-by: Yuchao Ma <yuchaom@codeaurora.org>