configure: Simplify fragile libdrm detection
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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.