msm_fb: display: Fix for compilation error with HDMI as primary

Proper variable is now used to avoid the compilation error. The
function where this error is seen is compiled only when the
HDMI is configured as PRMIARY.

Change-Id: I74a9693ceef49143412101a6a8360d2241f75e91
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>