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 64a4aff2 ("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

drivers/gpu/drm/gma500/psb_irq.c

@@ -206,7 +206,6 @@ static void psb_sgx_interrupt(struct drm_device *dev, u32 stat_1, u32 stat_2)
 {
 	struct drm_psb_private *dev_priv = dev->dev_private;
 	u32 val, addr;
-	int error = false;
 
 	if (stat_1 & _PSB_CE_TWOD_COMPLETE)
 		val = PSB_RSGX32(PSB_CR_2D_BLIT_STATUS);
@@ -241,7 +240,6 @@ static void psb_sgx_interrupt(struct drm_device *dev, u32 stat_1, u32 stat_2)
 
 			DRM_ERROR("\tMMU failing address is 0x%08x.\n",
 				  (unsigned int)addr);
-			error = true;
 		}
 	}