selftest: do not check for dri.pc in the headless test

runqemu (which the test also runs) already performs this check,
and does it better, as it additionally unsets a number of environment
variables that can confuse and misdirect host's pkg-config.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>