soc/intel/skylake: Factor out unnecessary if-else-block

soc/intel/skylake: Factor out unnecessary if-else-block

Move InternalGfx config option out of the if-else-block and replace the
left over config option IgdDvmt50PreAlloc by a ternary expression. Also,
adjust related code comments to fit the new logic of this code.

This changes the logic of the code, since InternalGfx is configured
first and IgdDvmt50PreAlloc depends on its value. The negation in the
ternary expression is removed to improve the readability.

Change-Id: I89ff17f4574a7ade228c1791f17ea072fb731775
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44369
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner