msm: mdss: dsi: propagate error on read command failures

When panel read command API fails, propagate the error up so that
the caller can take any necessary actions.

CRs-Fixed: 960973
Change-Id: I397f4d47c6343abbab12833a47b753dc3bdb98f0
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>