drm/i915/psr: Check for power state control capability.

eDP spec says - "If PSR/PSR2 is supported, the SET_POWER_CAPABLE bit in the
EDP_GENERAL_CAPABILITY_1 register (DPCD Address 00701h, bit d7) must be set
to 1."

Reject PSR on panels without this cap bit set as such panels cannot be
controlled via SET_POWER & SET_DP_PWR_VOLTAGE register and the DP source
needs to be able to do that for PSR.

Thanks to Nathan for debugging this.

Panel cap checks like this can be done just once, let's fix this
when PSR dpcd init movement lands.

Cc: Nathan D Ciobanu <nathan.d.ciobanu@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Tested-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227032723.15474-1-dhinakaran.pandiyan@intel.com