sdm: Fix avg luminance value for built-in displays

Primary panel don't have average luminance value. Hence, display
driver don't populate the same. Average luminance is initialized
to default value of zero in HAL. Add average luminance calculation
for primary display. This aligns with the external displays.

CRs-fixed: 2716028
Change-Id: I962adef4c8298e7247cc320cc5813f1060dad4c1
1 file changed