regulator: msm-gfx-ldo: Disable CX only in the error path

Disable the regulator vote on CX only if there is failure,
also check if the regulator handle is valid.

Change-Id: I8cbb8aad3c99d0a619dfeb2f68873c39316f0158
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>