gallium/dri: Remove dri2_format_mapping::cpp

I was suspicious that some entries in dri2_format_table (in dri_helpers.c) had this field set incorrectly. It seemed like DRM_FORMAT_ABGR16161616F and DRM_FORMAT_XBGR16161616F should have been 8 instead of 4. Upon digging I found that nothing uses the field. Fix code by removing it.