drm/i915: Get first crtc instead of PIPE_A crtc

intel_plane_fb_max_stride should return the max stride of
primary plane for first available pipe in intel device info
pipe_mask.
Similarly glk_force_audio_cdclk() should also use the first
available CRTC instead of pipe 'A' crtc to force the cdclk
changes.

changes since RFC:
- Introduced a helper to get first intel_crtc intel_get_first_crtc. [Ville]
v1:
- Used intel_get_first_crtc() instead of PIPE_A crtc in
  glk_force_audio_cdclk(). [Ville]

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224124004.26712-6-anshuman.gupta@intel.com