drm/i915/gvt: Handle EDP_PSR_IMR and EDP_PSR_IIR for BXT.

BXT supports EDP. However since GVT-g only simulate DP monitor
to guest and handles EDP_PSR_IMR and EDP_PSR_IIR as default MMIO
r/w. If guest r/w these IMR/IIR, GVT-g won't simulate the real
HW behavior and below warning is printed:
--------
Interrupt register 0x64838 is not zero: 0xffffffff
WARNING: CPU: 1 PID: 1 at drivers/gpu/drm/i915/i915_irq.c:161
gen3_assert_iir_is_zero+0x34/0xa0

Call Trace:
gen8_de_irq_postinstall+0xad/0x330
gen8_irq_postinstall+0x23/0x80
drm_irq_install+0xb5/0x130
i915_driver_load+0xafd/0xf70
--------
Since GVT-g won't simulate EDP to guest, always set EDP_PSR_IMR
and EDP_PSR_IIR IMR/IIR to 0.

Signed-off-by: Colin Xu <colin.xu@intel.com>

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>