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 <[e-mail]>
Cc: Daniel Vetter <[e-mail]>

drivers/gpu/drm/arc/arcpgu_drv.c

···
 	.driver_features = DRIVER_MODESET | DRIVER_GEM | DRIVER_PRIME |
 			   DRIVER_ATOMIC,
 	.lastclose = arcpgu_lastclose,
-	.name = "drm-arcpgu",
+	.name = "arcpgu",
 	.desc = "ARC PGU Controller",
 	.date = "20160219",
 	.major = 1,