Switch to the logic from x11-drivers/xf86-video-intel for detecting
whether graphics/libdrm with KMS enabled is installed. For one, the
WITH_KMS knob never real has been established as a global option
within the ports tree. Also, just using IGNORE here when libkms.so
not already exists does the wrong thing when graphics/libdrm generally
is not installed at this point of time, yet.
Approved by: bapt (mentor), maintainer