drm: Allow CAP_PRIME on !MODESET

vgem (and our igt tests using vgem) need this. I suspect etnaviv will
fare similarly.

v2. Make it build. Oops.

Fixes: d5264ed3 ("drm: Return -ENOTSUPP when called for KMS cap with a non-KMS driver")
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161207144939.22756-1-daniel.vetter@ffwll.ch