drm/i915/gvt: Handle values of EDP_PSR_IMR and EDP_PSR_IIR

GVT-g only simulates DP port for guest and leaves EDP_PSR_IMR
and EDP_PSR_IIR registers as default MMIO read/write.
So guest won't get expected initial values of these registers when
initializing the gpu driver, which results in following warning and logs.

--------
Interrupt register 0x64838 is not zero: 0xffffffff
WARNING: CPU: 1 PID: 157 at drivers/gpu/drm/i915/i915_irq.c:177
gen3_assert_iir_is_zero+0x38/0xa0

Call Trace:
gen8_de_irq_postinstall+0xa7/0x400
gen8_irq_postinstall+0x27/0x80
drm_irq_install+0xbc/0x140
i915_driver_load+0xa9d/0xd50
--------
Because GVT-g does not handle EDP(embedded DP) simulation for guests,
always set EDP_PSR_IMR and EDP_PSR_IIR to value 0.

Signed-off-by: Longhe Zheng <longhe.zheng@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>