surfaceflinger: simplify P3 support in RE

RE always knows how to convert sRGB/P3/BT2020 to P3.  The conversion
is enabled by SurfaceFlinger whenever the color mode is P3 (and when
hasWideColorDisplay is true).  This change simplifies or removes
some wide color state functions from RE, and moves the burden to
SurfaceFlinger.  It also changes GLES20RenderEngine::drawMesh to
inspect mDataSpace and mOutputDataSpace directly.

This allows us to enable the conversion in more cases more easily.
For example, we can choose to set the RE output dataspace to P3 even
when the display color mode is NATIVE.

Test: manual with UiBench
Change-Id: I5e701dd6b01764efbbab967dc4a26a4d008dfc09