drm/etnaviv: Remove useless error message

Fix the following coccicheck report:

drivers/gpu/drm/etnaviv/etnaviv_gpu.c:1775:2-9:
line 1775 is redundant because platform_get_irq() already prints an error

Remove dev_err() messages after platform_get_irq() failures.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Zihao Tang <tangzihao1@hisilicon.com>
Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>