drm/msm/mdp5: mdp5_crtc: Restore cursor state only if LM cursors are enabled

MDP5 on newer SoCs support cursor planes (i.e, cursor SSPPs). They are a
separate entity unlike the cursors within LM.

Do not try to restore the MDP5 LM cursor registers, or the corresponding
CTL bits if we are not using LM cursors.

Also, since we've introduced a new variable 'lm_cursor_enabled', we can
now use it to avoid creating a different sets of crtc_funcs for CRTCs
with LM cursors and CRTCs with cursor planes.

Fixes: "drm/msm/mdp5: restore cursor state when enabling crtc"
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>