My AMD GPU is not available in Cycle Render Preferences on version 2.8 I am using laptop with AMD Radeon HD 8670m GPU in it. I have latest drivers installed. But when I access Blender System Preferences it displays:

"NO COMPATIBLE GPU'S FOUND. CYCLES WILL RENDER IN CPU".

I used the same in 2.79 and I found  no problems. Please help me fix this issue. Thanks in advance.
Your graphics card doesn't meet the requirements for GPU rendering with Cycles as @DuaneDibbley already linked. The minimum requirements for the GPU are only for being able to run Blender at all.

For Blender 2.8 you will need GCN first generation or newer to run Blender. GCN second generation or newer is required for OpenCL GPU rendering.