arcpgu: Simplify driver name

This very minor change is still useful because it aligns
ARC PGU driver name with other DRM drivers and makes usage of
that driver name a bit easier.

For example in libdrm's test app we'll use "arcpgu" instead of
a bit more ugly "drm-arcpgu".

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>