configure: Simplify fragile libdrm detection

Merged Adam Jackson requested to merge ajax/xf86-video-qxl:fix-libdrm-detection into master

libdrm 2.4.46 always installs qxl_drm.h

The fragility here is if xserver doesn't sneak -I/usr/include/libdrm into your CFLAGS, we'd fail to find the header and disable DRM and KMS entirely. If we need that header path we should set it up, but we also don't, so just skip all this.

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