drm/gma500: remove set but not used variable 'error'

drm/gma500: remove set but not used variable 'error'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/gma500/psb_irq.c: In function psb_sgx_interrupt:
drivers/gpu/drm/gma500/psb_irq.c:210:6: warning: variable error set but not used [-Wunused-but-set-variable]

It is introduced by commit 64a4aff283ac ("drm/gma500:
Add support for SGX interrupts"), but never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1573828027-122323-3-git-send-email-zhengbin13@huawei.com