drivers/intel/gma/intel_dp.c: Fix printf type-specifier

'%02hx' is unsigned short, where as the argument is typed as
uint8_t and so '%02hhx' is actually correct here.

Found-by: Clang
Change-Id: I40c48dcecf12845f4708e511236184908e90fb56
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7428
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)