Make SkIPoint x() and y() constexpr
Make SkIPoint x() and y() constexpr

Fixes several GCC builds.

Change-Id: I9ad64b08d707a0e491590dea07617e89fe64d60d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/246176
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Auto-Submit: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia