backlight: fix pwm_bl.c when multiple PWM backlights exist

When multiple PWMs are used as backlights, the current code uses
pdev->name as the backlight name when registering, which will be
conflicting, use dev_name() instead.

Signed-off-by: Peter Edwards <sweetlilmre@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>