drm/msm/sde: fix crtc enable check

In SDE driver, existing check of crtc enabled is from enabled
of drm crtc object which is not reliable in atomic commit path
and this leads to improper clock calculation. This change fixes
crtc enable check by referring to member from sde crtc which
SDE driver is properly updating in crtc enable and disable.

Change-Id: I331339513307a3b1c87d8a59e390e90e6a9a0bdb
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>