drm: drm_err: Remove unnecessary __func__ argument

Removing the unnecessary drm_err __func__ argument by using
the equivalent %pf and __builtin_return_address(0) makes the
code smaller for every use of the DRM_ERROR macro.

For instance: (allmodconfig)

$ size drivers/gpu/drm/i915/i915.o*
   text	   data	    bss	    dec	    hex	filename
 922447	 193257	 296736	1412440	 158d58	drivers/gpu/drm/i915/i915.o.new
 928111	 193257	 296736	1418104	 15a378	drivers/gpu/drm/i915/i915.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>