Handle sRGB gralloc formats.

gralloc: recognize and allocate sRGB_{A,X}_8888
hwcomposer: punt sRGB formats to FRAMEBUFFER composition
- This would happen already due to the format-supported test, this
  change just avoids an unrecognized-format log warning from
  exynos5_format_to_bpp().

Bug: 10134664
Change-Id: I78b7791217cbde28d5bc7d02caf6a955960828ce