drm/hyperv: Add DRM driver for hyperv synthetic video device

DRM driver for hyperv synthetic video device, based on hyperv_fb
framebuffer driver. Also added config option "DRM_HYPERV" to enabled
this driver.

v2:
- Add support for gen2 VM
- Fixed review comments

v3:
- Split into multiple files as suggested by Thomas Zimmermann
- Fixed hibernation issue as suggested by Dexuan Cui
- Use ioremap_cache as suggested by Dexuan Cui
- Incorporated other review comments

v4:
- Fix bitrotted code
- Review comments
- Updated the copyright and license to match hyperv_fb

v5:
- Address review comments and rebased with drm-misc-next

v6:
- Minor code/comment improvement as suggested by Dexuan Cui

Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210527112230.1274-1-drawat.floss@gmail.com