drm: delete drm_pci.h

It's empty!

After more than 20 years of OS abstraction layer for pci devices, it's
kinda gone now.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403110610.2344842-2-daniel.vetter@ffwll.ch