drm/gma500: remove set but not used variables 'hist_reg'

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

drivers/gpu/drm/gma500/psb_irq.c: In function psb_irq_turn_off_dpst:
drivers/gpu/drm/gma500/psb_irq.c:473:6:
	warning: variable hist_reg set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191227114811.14907-1-chenzhou10@huawei.com