On Linux, they split the content into three headers:
1. <linux/compiler.h>
2. <linux/compiler_types.h>
3. <linux/compiler_attributes.h>

The first includes the second, which includes the third.

<linux/compiler_types.h> is also included on the compiler command line
by default! I added that to the compilation flags of the DRM drivers.
This allowed me to drop at least one:

    #ifdef __FreeBSD
    #include <linux/compiler.h>
    #endif

Note that our copy of <linux/compiler.h> contains definitions which are
not defined by Linux' <linux/compiler.h>. I left them alone.

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54495