linuxkpi: add kernel_fpu_begin/kernel_fpu_end

With newer AMD GPUs (>=Navi,Renoir) there is FPU context usage in the
amdgpu driver.
The kernel_fpu_begin/end implementations in drm did not even allow nested
begin-end blocks.

Submitted by: Greg V
Reviewed By: manu, hselasky
Differential Revision: https://reviews.freebsd.org/D28061