drm_hwcomposer: fix BufferInfoImagination build

Commit e9fbd8d6 ("drm_hwcomposer: Set return type to std::optional for
BufferInfoGetters") changed the BufferInfo variable name, but missed the case
defined under HAL_PIXEL_FORMAT_BGRX_8888. This results in build failures.

This patch fixes the build issue by completing the variable renaming.

Fixes: e9fbd8d6 ("drm_hwcomposer: Set return type to std::optional for BufferInfoGetters")

Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>