A $5.4-million jury award granted two years ago to a retarded, blind, deaf and quadriplegic San Gabriel girl has been reduced to $2.7 million by the 2nd District Court of Appeal. The court ruled that a 1975 California statue limited the pain and suffering damages that 12-year-old Brandi Mendez could receive. When she was 5 1/2 months old, Mendez was rushed to Community Hospital of San Gabriel on Easter weekend, 1974, with a temperature of 106.5 degrees. Emergency room personnel gave her cooling baths and sent her home, failing to diagnose that she had meningitis. Two days later she was returned to the hospital with convulsions. The treatment delay left her with severe and permanent brain damage. The appellate court ruled that it made no difference that the girl's injury occured before the state law was enacted, and lowered the $3-million pain and suffering award to the law's $250,000 maximum. The $2.4-million she had received for medical care and lost future earnings was left intact.